diff --git a/src/Data/String/CodePoints.purs b/src/Data/String/CodePoints.purs
index 82d263c..7d0f528 100644
--- a/src/Data/String/CodePoints.purs
+++ b/src/Data/String/CodePoints.purs
@@ -286,11 +286,23 @@ lastIndexOf p s = (\i -> length (CU.take i s)) <$> CU.lastIndexOf p s
 
 -- | Returns the number of code points preceding the first match of the given
 -- | pattern in the string. Pattern matches following the given index will be
--- | ignored. Returns Nothing when no matches are found.
+-- | ignored.
+-- |
+-- | Giving a negative index is equivalent to giving 0 and giving an index
+-- | greater than the number of code points in the string is equivalent to
+-- | searching in the whole string.
+-- |
+-- | Returns Nothing when no matches are found.
 -- |
 -- | ```purescript
+-- | >>> lastIndexOf' (Pattern "𝐀") (-1) "b 𝐀𝐀 c 𝐀"
+-- | Nothing
+-- | >>> lastIndexOf' (Pattern "𝐀") 0 "b 𝐀𝐀 c 𝐀"
+-- | Nothing
 -- | >>> lastIndexOf' (Pattern "𝐀") 5 "b 𝐀𝐀 c 𝐀"
 -- | Just 3
+-- | >>> lastIndexOf' (Pattern "𝐀") 8 "b 𝐀𝐀 c 𝐀"
+-- | Just 7
 -- | >>> lastIndexOf' (Pattern "o") 5 "b 𝐀𝐀 c 𝐀"
 -- | Nothing
 -- | ```
diff --git a/src/Data/String/CodeUnits.js b/src/Data/String/CodeUnits.js
index 870b96f..6017fd3 100644
--- a/src/Data/String/CodeUnits.js
+++ b/src/Data/String/CodeUnits.js
@@ -83,7 +83,6 @@ exports._lastIndexOfStartingAt = function (just) {
     return function (x) {
       return function (startAt) {
         return function (s) {
-          if (startAt < 0 || startAt > s.length) return nothing;
           var i = s.lastIndexOf(x, startAt);
           return i === -1 ? nothing : just(i);
         };
diff --git a/src/Data/String/CodeUnits.purs b/src/Data/String/CodeUnits.purs
index cc919e0..dfa0482 100644
--- a/src/Data/String/CodeUnits.purs
+++ b/src/Data/String/CodeUnits.purs
@@ -215,14 +215,21 @@ foreign import _lastIndexOf
   -> Maybe Int
 
 -- | Returns the index of the last occurrence of the pattern in the
--- | given string, starting at the specified index
--- | and searching backwards towards the beginning of the string.
+-- | given string, starting at the specified index and searching
+-- | backwards towards the beginning of the string.
+-- |
+-- | Starting at a negative index is equivalent to starting at 0 and
+-- | starting at an index greater than the string length is equivalent
+-- | to searching in the whole string.
+-- |
 -- | Returns `Nothing` if there is no match.
 -- |
 -- | ```purescript
+-- | lastIndexOf' (Pattern "a") (-1) "ababa" == Just 0
 -- | lastIndexOf' (Pattern "a") 1 "ababa" == Just 0
 -- | lastIndexOf' (Pattern "a") 3 "ababa" == Just 2
 -- | lastIndexOf' (Pattern "a") 4 "ababa" == Just 4
+-- | lastIndexOf' (Pattern "a") 5 "ababa" == Just 4
 -- | ```
 -- |
 lastIndexOf' :: Pattern -> Int -> String -> Maybe Int
diff --git a/src/Data/String/NonEmpty/CodeUnits.purs b/src/Data/String/NonEmpty/CodeUnits.purs
index d22e180..af3de43 100644
--- a/src/Data/String/NonEmpty/CodeUnits.purs
+++ b/src/Data/String/NonEmpty/CodeUnits.purs
@@ -159,14 +159,21 @@ lastIndexOf :: Pattern -> NonEmptyString -> Maybe Int
 lastIndexOf = liftS <<< CU.lastIndexOf
 
 -- | Returns the index of the last occurrence of the pattern in the
--- | given string, starting at the specified index
--- | and searching backwards towards the beginning of the string.
+-- | given string, starting at the specified index and searching
+-- | backwards towards the beginning of the string.
+-- |
+-- | Starting at a negative index is equivalent to starting at 0 and
+-- | starting at an index greater than the string length is equivalent
+-- | to searching in the whole string.
+-- |
 -- | Returns `Nothing` if there is no match.
 -- |
 -- | ```purescript
+-- | lastIndexOf' (Pattern "a") (-1) (NonEmptyString "ababa") == Just 0
 -- | lastIndexOf' (Pattern "a") 1 (NonEmptyString "ababa") == Just 0
 -- | lastIndexOf' (Pattern "a") 3 (NonEmptyString "ababa") == Just 2
 -- | lastIndexOf' (Pattern "a") 4 (NonEmptyString "ababa") == Just 4
+-- | lastIndexOf' (Pattern "a") 5 (NonEmptyString "ababa") == Just 4
 -- | ```
 lastIndexOf' :: Pattern -> Int -> NonEmptyString -> Maybe Int
 lastIndexOf' pat = liftS <<< CU.lastIndexOf' pat
diff --git a/test/Test/Data/String/CodePoints.purs b/test/Test/Data/String/CodePoints.purs
index 310321b..587ec89 100644
--- a/test/Test/Data/String/CodePoints.purs
+++ b/test/Test/Data/String/CodePoints.purs
@@ -366,6 +366,10 @@ testStringCodePoints = do
     { actual: SCP.lastIndexOf' (Pattern str) 1 str
     , expected: Just 0
     }
+  assertEqual
+    { actual: SCP.lastIndexOf' (Pattern "a") (-1) str
+    , expected: Just 0
+    }
   assertEqual
     { actual: SCP.lastIndexOf' (Pattern "a") 0 str
     , expected: Just 0
@@ -374,6 +378,10 @@ testStringCodePoints = do
     { actual: SCP.lastIndexOf' (Pattern "a") 7 str
     , expected: Just 0
     }
+  assertEqual
+    { actual: SCP.lastIndexOf' (Pattern "a") (SCP.length str) str
+    , expected: Just 0
+    }
   assertEqual
     { actual: SCP.lastIndexOf' (Pattern "z") 0 str
     , expected: Nothing
diff --git a/test/Test/Data/String/CodeUnits.purs b/test/Test/Data/String/CodeUnits.purs
index 249a639..9cf010b 100644
--- a/test/Test/Data/String/CodeUnits.purs
+++ b/test/Test/Data/String/CodeUnits.purs
@@ -270,7 +270,7 @@ testStringCodeUnits = do
     }
   assertEqual
     { actual: SCU.lastIndexOf' (Pattern "") (-1) "ab"
-    , expected: Nothing
+    , expected: Just 0
     }
   assertEqual
     { actual: SCU.lastIndexOf' (Pattern "") 0 "ab"
@@ -286,7 +286,7 @@ testStringCodeUnits = do
     }
   assertEqual
     { actual: SCU.lastIndexOf' (Pattern "") 3 "ab"
-    , expected: Nothing
+    , expected: Just 2
     }
   assertEqual
     { actual: SCU.lastIndexOf' (Pattern "bc") 0 "abcd"
diff --git a/test/Test/Data/String/NonEmpty/CodeUnits.purs b/test/Test/Data/String/NonEmpty/CodeUnits.purs
index 0e78ee8..9a33b0c 100644
--- a/test/Test/Data/String/NonEmpty/CodeUnits.purs
+++ b/test/Test/Data/String/NonEmpty/CodeUnits.purs
@@ -277,7 +277,7 @@ testNonEmptyStringCodeUnits = do
   log "lastIndexOf'"
   assertEqual
     { actual: NESCU.lastIndexOf' (Pattern "") (-1) (nes (Proxy :: Proxy "ab"))
-    , expected: Nothing
+    , expected: Just 0
     }
   assertEqual
     { actual: NESCU.lastIndexOf' (Pattern "") 0 (nes (Proxy :: Proxy "ab"))
@@ -293,7 +293,7 @@ testNonEmptyStringCodeUnits = do
     }
   assertEqual
     { actual: NESCU.lastIndexOf' (Pattern "") 3 (nes (Proxy :: Proxy "ab"))
-    , expected: Nothing
+    , expected: Just 2
     }
   assertEqual
     { actual: NESCU.lastIndexOf' (Pattern "bc") 0 (nes (Proxy :: Proxy "abcd"))