diff --git a/src/vm/wren_core.c b/src/vm/wren_core.c
index a0f96caa..de55506c 100644
--- a/src/vm/wren_core.c
+++ b/src/vm/wren_core.c
@@ -942,7 +942,7 @@ DEF_PRIMITIVE(num_fromString)
   double number = strtod(string->value, &end);
 
   // Skip past any trailing whitespace.
-  while (*end != '\0' && isspace(*end)) end++;
+  while (*end != '\0' && isspace((unsigned char) *end)) end++;
 
   if (errno == ERANGE)
   {