diff --git a/stdlib/internal/str.codon b/stdlib/internal/str.codon
index d10146b7..274da890 100644
--- a/stdlib/internal/str.codon
+++ b/stdlib/internal/str.codon
@@ -591,7 +591,7 @@ class str:
         fill = width - len(self)
         p = zf.ptr
 
-        if p[fill] == plus or p[fill] == minus:
+        if len(self) > 0 and p[fill] == plus or p[fill] == minus:
             p[0] = p[fill]
             p[fill] = zero