diff --git a/scripts/fuzz_opt.py b/scripts/fuzz_opt.py
index 171a246d255..6705d79526c 100755
--- a/scripts/fuzz_opt.py
+++ b/scripts/fuzz_opt.py
@@ -584,7 +584,7 @@ def fix_double(x):
 
     # JS prints i31 as just a number, so change "i31ref(N)" (which C++ emits)
     # to "N".
-    out = re.sub(r'i31ref\((\d+)\)', r'\1', out)
+    out = re.sub(r'i31ref\((-?\d+)\)', r'\1', out)
 
     lines = out.splitlines()
     for i in range(len(lines)):