diff --git a/core/src/main/java/org/jruby/util/StringSupport.java b/core/src/main/java/org/jruby/util/StringSupport.java
index ca6e79921e8..8b1e9f81838 100644
--- a/core/src/main/java/org/jruby/util/StringSupport.java
+++ b/core/src/main/java/org/jruby/util/StringSupport.java
@@ -2505,8 +2505,8 @@ public static int multiByteCasecmp(Encoding enc, ByteList value, ByteList otherV
 
             final int cl, ocl;
             if (Encoding.isAscii(c) && Encoding.isAscii(oc)) {
-                int dc = AsciiTables.ToUpperCaseTable[c];
-                int odc = AsciiTables.ToUpperCaseTable[oc];
+                int dc = AsciiTables.ToLowerCaseTable[c];
+                int odc = AsciiTables.ToLowerCaseTable[oc];
                 if (dc != odc) return dc < odc ? -1 : 1;
 
                 if (enc.isAsciiCompatible()) {