diff --git a/src/Gren/Kernel/String.js b/src/Gren/Kernel/String.js
index 6d28f51..9b83895 100644
--- a/src/Gren/Kernel/String.js
+++ b/src/Gren/Kernel/String.js
@@ -214,7 +214,7 @@ function _String_toInt(str) {
     if (code < 0x30 || 0x39 < code) {
       return __Maybe_Nothing;
     }
-    total = 10 * total + code - 0x30;
+    total = 10 * total + (code - 0x30);
   }
 
   return i == start
diff --git a/tests/src/Test/String.gren b/tests/src/Test/String.gren
index c98a660..4fb0b63 100644
--- a/tests/src/Test/String.gren
+++ b/tests/src/Test/String.gren
@@ -76,6 +76,10 @@ tests =
             , badInt "1e31"
             , badInt "123a"
             , goodInt "0123" 123
+            , goodInt "9007199254740944" 9007199254740944
+            , goodInt "9007199254740945" 9007199254740945
+            , goodInt "9007199254740991" 9007199254740991
+            , goodInt "-9007199254740991" -9007199254740991
             , badInt "0x001A"
             , badInt "0x001a"
             , badInt "0xBEEF"