Index: src/OFString.m
==================================================================
--- src/OFString.m
+++ src/OFString.m
@@ -2333,11 +2333,11 @@
 
 	/* Check if there are any invalid chars left */
 	if (endPointer != NULL)
 		for (; *endPointer != '\0'; endPointer++)
 			/* Use isspace since strtoll uses the same. */
-			if (!isspace(*endPointer))
+			if (!isspace((unsigned char)*endPointer))
 				@throw [OFInvalidFormatException exception];
 
 	objc_autoreleasePoolPop(pool);
 
 	return value;
@@ -2354,11 +2354,11 @@
 	const char *UTF8String = self.UTF8String;
 	char *endPointer = NULL;
 	unsigned long long value;
 
 	/* Use isspace since strtoull uses the same. */
-	while (isspace(*UTF8String))
+	while (isspace((unsigned char)*UTF8String))
 		UTF8String++;
 
 	if (*UTF8String == '-')
 		@throw [OFInvalidFormatException exception];
 
@@ -2369,11 +2369,11 @@
 
 	/* Check if there are any invalid chars left */
 	if (endPointer != NULL)
 		for (; *endPointer != '\0'; endPointer++)
 			/* Use isspace since strtoull uses the same. */
-			if (!isspace(*endPointer))
+			if (!isspace((unsigned char)*endPointer))
 				@throw [OFInvalidFormatException exception];
 
 	objc_autoreleasePoolPop(pool);
 
 	return value;
@@ -2417,11 +2417,11 @@
 
 	/* Check if there are any invalid chars left */
 	if (endPointer != NULL)
 		for (; *endPointer != '\0'; endPointer++)
 			/* Use isspace since strtof uses the same. */
-			if (!isspace(*endPointer))
+			if (!isspace((unsigned char)*endPointer))
 				@throw [OFInvalidFormatException exception];
 
 	objc_autoreleasePoolPop(pool);
 
 	return value;
@@ -2465,11 +2465,11 @@
 
 	/* Check if there are any invalid chars left */
 	if (endPointer != NULL)
 		for (; *endPointer != '\0'; endPointer++)
 			/* Use isspace since strtod uses the same. */
-			if (!isspace(*endPointer))
+			if (!isspace((unsigned char)*endPointer))
 				@throw [OFInvalidFormatException exception];
 
 	objc_autoreleasePoolPop(pool);
 
 	return value;