diff --git a/script.go b/script.go
index 3488d184..9c98a703 100644
--- a/script.go
+++ b/script.go
@@ -330,7 +330,9 @@ func parseScriptTemplate(script []byte, opcodemap map[byte]*opcode) ([]parsedOpc
 				return nil, err
 			}
 			off = i + 1 - op.length // beginning of data
-			if int(l) > len(script[off:]) {
+			// Disallow entries that do not fit script or were
+			// sign extended.
+			if int(l) >= len(script[off:]) || int(l) < 0 {
 				return retScript, StackErrShortScript
 			}
 			pop.data = script[off : off+int(l)]