diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 5fc8d2da..4ad4df4c 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -578,7 +578,7 @@ class bigint {
   }
 
   void assign(uint64_t n) {
-    int num_bigits = 0;
+    size_t num_bigits = 0;
     do {
       bigits_[num_bigits++] = n & ~bigit(0);
       n >>= bigit_bits;