diff --git a/src/main/java/org/squiddev/cobalt/lib/StringLib.java b/src/main/java/org/squiddev/cobalt/lib/StringLib.java
index 882ab94c..ff23b827 100644
--- a/src/main/java/org/squiddev/cobalt/lib/StringLib.java
+++ b/src/main/java/org/squiddev/cobalt/lib/StringLib.java
@@ -663,7 +663,7 @@ private static int posrelat(int pos, int len) {
 	private static final byte[] CHAR_TABLE;
 
 	public static boolean isWhitespace(byte b) {
-		return (CHAR_TABLE[b] & MASK_SPACE) != 0;
+		return (CHAR_TABLE[b & 0xFF] & MASK_SPACE) != 0;
 	}
 
 	static {
diff --git a/src/test/resources/assert/stringIssues.lua b/src/test/resources/assert/stringIssues.lua
index 5200e256..de488c9a 100644
--- a/src/test/resources/assert/stringIssues.lua
+++ b/src/test/resources/assert/stringIssues.lua
@@ -96,3 +96,6 @@ assert(tostring(1e39) == "1e+39", "Got " .. tostring(1e39))
 -- Tiny bits of string concatination
 local x, y, z = "foo", "bar", "baz"
 assert(x .. y .. z == table.concat { x, y, z })
+
+-- Whitespace on large characters
+assert(tonumber(("\128")) == nil)