diff --git a/CHANGELOG.md b/CHANGELOG.md
index e04806f4..667b4b22 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,8 @@
 
 * Improve some error messages for edge-case parse failures.
 
+* Don't crash when writing `Infinity` in JS mode.
+
 ## 1.26.10
 
 * Fixes a bug where two adjacent combinators could cause an error.
diff --git a/lib/src/util/number.dart b/lib/src/util/number.dart
index 02aaae43..da9679bc 100644
--- a/lib/src/util/number.dart
+++ b/lib/src/util/number.dart
@@ -39,6 +39,8 @@ bool fuzzyGreaterThanOrEquals(num number1, num number2) =>
 
 /// Returns whether [number] is [fuzzyEquals] to an integer.
 bool fuzzyIsInt(num number) {
+  // Check this before is int to work around dart-lang/sdk#43325.
+  if (number.isInfinite || number.isNaN) return false;
   if (number is int) return true;
 
   // Check against 0.5 rather than 0.0 so that we catch numbers that are both
diff --git a/test/output_test.dart b/test/output_test.dart
index 582e59d6..8be030f7 100644
--- a/test/output_test.dart
+++ b/test/output_test.dart
@@ -44,6 +44,11 @@ void main() {
     });
 
     group("for floating-point numbers", () {
+      test("Infinity", () {
+        expect(compileString("a {b: 1e999}"),
+            equalsIgnoringWhitespace("a { b: Infinity; }"));
+      });
+
       test(">= 1e21", () {
         expect(compileString("a {b: 1.01e21}"),
             equalsIgnoringWhitespace("a { b: 101${'0' * 19}; }"));