diff --git a/math.c b/math.c
index 849d822..e05980b 100644
--- a/math.c
+++ b/math.c
@@ -459,11 +459,14 @@ static VALUE
 math_log(int argc, const VALUE *argv, VALUE obj)
 {
     VALUE x, base;
-    double d;
+    double d0, d;

     rb_scan_args(argc, argv, "11", &x, &base);
     d = math_log1(x);
     if (argc == 2) {
+        Need_Float(base);
+        d0 = RFLOAT_VALUE(base);
+        if (d0 == 0.0 || d0 == 1.0) domain_error("log");
         d /= math_log1(base);
     }
     return DBL2NUM(d);