diff --git a/src/execution/__tests__/executor-test.js b/src/execution/__tests__/executor-test.js
index 4d2fdab48f..fa293df123 100644
--- a/src/execution/__tests__/executor-test.js
+++ b/src/execution/__tests__/executor-test.js
@@ -32,7 +32,7 @@ describe('Execute: Handles basic execution tasks', () => {
     });
 
     // $DisableFlowOnNegativeTest
-    expect(() => execute({ schema })).to.throw('Must provide document');
+    expect(() => execute({ schema })).to.throw('Must provide document.');
   });
 
   it('throws if no schema is provided', () => {
diff --git a/src/execution/execute.js b/src/execution/execute.js
index d13590d261..fe80d6c980 100644
--- a/src/execution/execute.js
+++ b/src/execution/execute.js
@@ -251,7 +251,7 @@ export function assertValidExecutionArguments(
   document: DocumentNode,
   rawVariableValues: ?{ +[variable: string]: mixed, ... },
 ): void {
-  devAssert(document, 'Must provide document');
+  devAssert(document, 'Must provide document.');
 
   // If the schema used for execution is invalid, throw an error.
   assertValidSchema(schema);
diff --git a/src/jsutils/invariant.js b/src/jsutils/invariant.js
index a53db54a8e..b32e04a84f 100644
--- a/src/jsutils/invariant.js
+++ b/src/jsutils/invariant.js
@@ -4,7 +4,7 @@ export default function invariant(condition: mixed, message?: string): void {
   const booleanCondition = Boolean(condition);
   if (!booleanCondition) {
     throw new Error(
-      message != null ? message : 'Unexpected invariant triggered',
+      message != null ? message : 'Unexpected invariant triggered.',
     );
   }
 }
diff --git a/src/language/__tests__/parser-test.js b/src/language/__tests__/parser-test.js
index 61dc49792d..a8b8bb9656 100644
--- a/src/language/__tests__/parser-test.js
+++ b/src/language/__tests__/parser-test.js
@@ -23,12 +23,12 @@ function expectSyntaxError(text) {
 describe('Parser', () => {
   it('asserts that a source to parse was provided', () => {
     // $DisableFlowOnNegativeTest
-    expect(() => parse()).to.throw('Must provide Source. Received: undefined');
+    expect(() => parse()).to.throw('Must provide Source. Received: undefined.');
   });
 
   it('asserts that an invalid source to parse was provided', () => {
     // $DisableFlowOnNegativeTest
-    expect(() => parse({})).to.throw('Must provide Source. Received: {}');
+    expect(() => parse({})).to.throw('Must provide Source. Received: {}.');
   });
 
   it('parse provides useful errors', () => {
diff --git a/src/language/__tests__/printer-test.js b/src/language/__tests__/printer-test.js
index 1df337b08c..aab7daa182 100644
--- a/src/language/__tests__/printer-test.js
+++ b/src/language/__tests__/printer-test.js
@@ -27,7 +27,7 @@ describe('Printer: Query document', () => {
     const badAST = { random: 'Data' };
     // $DisableFlowOnNegativeTest
     expect(() => print(badAST)).to.throw(
-      'Invalid AST Node: { random: "Data" }',
+      'Invalid AST Node: { random: "Data" }.',
     );
   });
 
diff --git a/src/language/__tests__/schema-printer-test.js b/src/language/__tests__/schema-printer-test.js
index 3993f1bf6d..3862ff53db 100644
--- a/src/language/__tests__/schema-printer-test.js
+++ b/src/language/__tests__/schema-printer-test.js
@@ -23,7 +23,7 @@ describe('Printer: SDL document', () => {
     const badAST = { random: 'Data' };
     // $DisableFlowOnNegativeTest
     expect(() => print(badAST)).to.throw(
-      'Invalid AST Node: { random: "Data" }',
+      'Invalid AST Node: { random: "Data" }.',
     );
   });
 
diff --git a/src/language/parser.js b/src/language/parser.js
index b877df7102..82e120ce2a 100644
--- a/src/language/parser.js
+++ b/src/language/parser.js
@@ -171,7 +171,7 @@ class Parser {
     const sourceObj = typeof source === 'string' ? new Source(source) : source;
     devAssert(
       sourceObj instanceof Source,
-      `Must provide Source. Received: ${inspect(sourceObj)}`,
+      `Must provide Source. Received: ${inspect(sourceObj)}.`,
     );
 
     this._lexer = new Lexer(sourceObj);
diff --git a/src/language/source.js b/src/language/source.js
index 2e2127a377..68c117f648 100644
--- a/src/language/source.js
+++ b/src/language/source.js
@@ -27,11 +27,11 @@ export class Source {
     this.locationOffset = locationOffset || { line: 1, column: 1 };
     devAssert(
       this.locationOffset.line > 0,
-      'line in locationOffset is 1-indexed and must be positive',
+      'line in locationOffset is 1-indexed and must be positive.',
     );
     devAssert(
       this.locationOffset.column > 0,
-      'column in locationOffset is 1-indexed and must be positive',
+      'column in locationOffset is 1-indexed and must be positive.',
     );
   }
 }
diff --git a/src/language/visitor.js b/src/language/visitor.js
index 9bccb1bc62..ef3d533fee 100644
--- a/src/language/visitor.js
+++ b/src/language/visitor.js
@@ -294,7 +294,7 @@ export function visit(
     let result;
     if (!Array.isArray(node)) {
       if (!isNode(node)) {
-        throw new Error('Invalid AST Node: ' + inspect(node));
+        throw new Error(`Invalid AST Node: ${inspect(node)}.`);
       }
       const visitFn = getVisitFn(visitor, node.kind, isLeaving);
       if (visitFn) {
diff --git a/src/subscription/__tests__/subscribe-test.js b/src/subscription/__tests__/subscribe-test.js
index 94d7766cc3..a9952b3343 100644
--- a/src/subscription/__tests__/subscribe-test.js
+++ b/src/subscription/__tests__/subscribe-test.js
@@ -337,13 +337,13 @@ describe('Subscription Initialization Phase', () => {
     await expectPromiseToThrow(
       // $DisableFlowOnNegativeTest
       () => subscribe(emailSchema, null),
-      'Must provide document',
+      'Must provide document.',
     );
 
     await expectPromiseToThrow(
       // $DisableFlowOnNegativeTest
       () => subscribe({ schema: emailSchema }),
-      'Must provide document',
+      'Must provide document.',
     );
   });
 
@@ -386,7 +386,7 @@ describe('Subscription Initialization Phase', () => {
 
     await expectPromiseToThrow(
       () => createSubscription(pubsub, invalidEmailSchema),
-      'Subscription field must return Async Iterable. Received: "test"',
+      'Subscription field must return Async Iterable. Received: "test".',
     );
   });
 
diff --git a/src/subscription/subscribe.js b/src/subscription/subscribe.js
index 5252532675..ac90c3e1c5 100644
--- a/src/subscription/subscribe.js
+++ b/src/subscription/subscribe.js
@@ -283,9 +283,10 @@ export function createSourceEventStream(
         // Note: isAsyncIterable above ensures this will be correct.
         return ((eventStream: any): AsyncIterable<mixed>);
       }
+
       throw new Error(
-        'Subscription field must return Async Iterable. Received: ' +
-          inspect(eventStream),
+        'Subscription field must return Async Iterable. ' +
+          `Received: ${inspect(eventStream)}.`,
       );
     });
   } catch (error) {
diff --git a/src/type/__tests__/definition-test.js b/src/type/__tests__/definition-test.js
index 64e010d127..3d979ce2cd 100644
--- a/src/type/__tests__/definition-test.js
+++ b/src/type/__tests__/definition-test.js
@@ -303,7 +303,7 @@ describe('Type System: Objects', () => {
       },
     });
     expect(() => objType.getFields()).to.throw(
-      'SomeObject.f field config must be an object',
+      'SomeObject.f field config must be an object.',
     );
   });
 
diff --git a/src/type/definition.js b/src/type/definition.js
index 0970774d8e..e4d2542c2a 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -765,7 +765,7 @@ function defineFieldMap<TSource, TContext>(
   return mapValue(fieldMap, (fieldConfig, fieldName) => {
     devAssert(
       isPlainObj(fieldConfig),
-      `${config.name}.${fieldName} field config must be an object`,
+      `${config.name}.${fieldName} field config must be an object.`,
     );
     devAssert(
       !('isDeprecated' in fieldConfig),
diff --git a/src/utilities/__tests__/assertValidName-test.js b/src/utilities/__tests__/assertValidName-test.js
index c08b1f723b..8eb60dbd6d 100644
--- a/src/utilities/__tests__/assertValidName-test.js
+++ b/src/utilities/__tests__/assertValidName-test.js
@@ -14,7 +14,7 @@ describe('assertValidName()', () => {
 
   it('throws for non-strings', () => {
     // $DisableFlowOnNegativeTest
-    expect(() => assertValidName({})).to.throw(/Expected string/);
+    expect(() => assertValidName({})).to.throw('Expected name to be a string.');
   });
 
   it('throws for names with invalid characters', () => {
diff --git a/src/utilities/__tests__/stripIgnoredCharacters-test.js b/src/utilities/__tests__/stripIgnoredCharacters-test.js
index f410261e58..12c895a86c 100644
--- a/src/utilities/__tests__/stripIgnoredCharacters-test.js
+++ b/src/utilities/__tests__/stripIgnoredCharacters-test.js
@@ -109,14 +109,14 @@ describe('stripIgnoredCharacters', () => {
   it('asserts that a source was provided', () => {
     // $DisableFlowOnNegativeTest
     expect(() => stripIgnoredCharacters()).to.throw(
-      'Must provide string or Source. Received: undefined',
+      'Must provide string or Source. Received: undefined.',
     );
   });
 
   it('asserts that a valid source was provided', () => {
     // $DisableFlowOnNegativeTest
     expect(() => stripIgnoredCharacters({})).to.throw(
-      'Must provide string or Source. Received: {}',
+      'Must provide string or Source. Received: {}.',
     );
   });
 
diff --git a/src/utilities/assertValidName.js b/src/utilities/assertValidName.js
index 8622236dd2..032e798cf2 100644
--- a/src/utilities/assertValidName.js
+++ b/src/utilities/assertValidName.js
@@ -25,7 +25,7 @@ export function isValidNameError(
   name: string,
   node?: ASTNode | void,
 ): GraphQLError | void {
-  devAssert(typeof name === 'string', 'Expected string');
+  devAssert(typeof name === 'string', 'Expected name to be a string.');
   if (name.length > 1 && name[0] === '_' && name[1] === '_') {
     return new GraphQLError(
       `Name "${name}" must not begin with "__", which is reserved by GraphQL introspection.`,
diff --git a/src/utilities/astFromValue.js b/src/utilities/astFromValue.js
index e4972960a4..9bf8501175 100644
--- a/src/utilities/astFromValue.js
+++ b/src/utilities/astFromValue.js
@@ -138,7 +138,7 @@ export function astFromValue(value: mixed, type: GraphQLInputType): ?ValueNode {
       };
     }
 
-    throw new TypeError(`Cannot convert value to AST: ${inspect(serialized)}`);
+    throw new TypeError(`Cannot convert value to AST: ${inspect(serialized)}.`);
   }
 
   // Not reachable. All possible input types have been considered.
diff --git a/src/utilities/buildASTSchema.js b/src/utilities/buildASTSchema.js
index 4fbb763ffa..337efd125d 100644
--- a/src/utilities/buildASTSchema.js
+++ b/src/utilities/buildASTSchema.js
@@ -118,7 +118,7 @@ export function buildASTSchema(
 ): GraphQLSchema {
   devAssert(
     documentAST && documentAST.kind === Kind.DOCUMENT,
-    'Must provide valid Document AST',
+    'Must provide valid Document AST.',
   );
 
   if (!options || !(options.assumeValid || options.assumeValidSDL)) {
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 74383e74d2..d2391939f6 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -99,7 +99,7 @@ export function extendSchema(
 
   devAssert(
     documentAST && documentAST.kind === Kind.DOCUMENT,
-    'Must provide valid Document AST',
+    'Must provide valid Document AST.',
   );
 
   if (!options || !(options.assumeValid || options.assumeValidSDL)) {
@@ -211,7 +211,7 @@ export function extendSchema(
   function replaceDirectives(
     directives: $ReadOnlyArray<GraphQLDirective>,
   ): Array<GraphQLDirective> {
-    devAssert(directives, 'schema must have default directives');
+    devAssert(directives, 'schema must have default directives.');
 
     return directives.map(directive => {
       const config = directive.toConfig();
diff --git a/src/utilities/stripIgnoredCharacters.js b/src/utilities/stripIgnoredCharacters.js
index e6ad305a1b..2b2036b109 100644
--- a/src/utilities/stripIgnoredCharacters.js
+++ b/src/utilities/stripIgnoredCharacters.js
@@ -66,7 +66,7 @@ export function stripIgnoredCharacters(source: string | Source): string {
   const sourceObj = typeof source === 'string' ? new Source(source) : source;
   if (!(sourceObj instanceof Source)) {
     throw new TypeError(
-      `Must provide string or Source. Received: ${inspect(sourceObj)}`,
+      `Must provide string or Source. Received: ${inspect(sourceObj)}.`,
     );
   }
 
diff --git a/src/validation/validate.js b/src/validation/validate.js
index 2fb7b83896..6adc314d1c 100644
--- a/src/validation/validate.js
+++ b/src/validation/validate.js
@@ -43,7 +43,7 @@ export function validate(
   typeInfo?: TypeInfo = new TypeInfo(schema),
   options?: {| maxErrors?: number |} = { maxErrors: undefined },
 ): $ReadOnlyArray<GraphQLError> {
-  devAssert(documentAST, 'Must provide document');
+  devAssert(documentAST, 'Must provide document.');
   // If the schema used for validation is invalid, throw an error.
   assertValidSchema(schema);