diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index f48f3edb18..e560c19c06 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -2704,7 +2704,7 @@ rb_uint64_convert_to_BigDecimal(uint64_t uval, RB_UNUSED_VAR(size_t digs), int r
         vp->frac[0] = (DECDIG)uval;
     }
     else {
-        const size_t len = (size_t)ceil(log10(uval) / BASE_FIG);
+        const size_t len = (size_t)ceil(log10((double)uval) / BASE_FIG);
 
         vp = VpAllocReal(len);
         vp->MaxPrec = len;