diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index 395c502d6151..6d4f3a1daa0c 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -2299,6 +2299,8 @@ test "float.decimal" {
     try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 1.40130e-45)});
     try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 9.999960e-40)});
     try expectFmt("f64: 10000000000000.00", "f64: {d:.2}", .{@as(f64, 9999999999999.999)});
+    try expectFmt("f64: 10000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e37)});
+    try expectFmt("f64: 100000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e38)});
 }
 
 test "float.libc.sanity" {
diff --git a/lib/std/fmt/errol.zig b/lib/std/fmt/errol.zig
index e98c23f6ec6b..29dd2b7a631f 100644
--- a/lib/std/fmt/errol.zig
+++ b/lib/std/fmt/errol.zig
@@ -106,7 +106,10 @@ fn errol3u(val: f64, buffer: []u8) FloatDecimal {
     } else if (val >= 16.0 and val < 9.007199254740992e15) {
         return errolFixed(val, buffer);
     }
+    return errolSlow(val, buffer);
+}
 
+fn errolSlow(val: f64, buffer: []u8) FloatDecimal {
     // normalize the midpoint
 
     const e = math.frexp(val).exponent;
@@ -336,7 +339,9 @@ fn errolInt(val: f64, buffer: []u8) FloatDecimal {
     var buf_index = u64toa(m64, buffer) - 1;
 
     if (mi != 0) {
-        buffer[buf_index - 1] += @boolToInt(buffer[buf_index] >= '5');
+        const round_up = buffer[buf_index] >= '5';
+        if (buf_index == 0 or (round_up and buffer[buf_index - 1] == '9')) return errolSlow(val, buffer);
+        buffer[buf_index - 1] += @boolToInt(round_up);
     } else {
         buf_index += 1;
     }