diff --git a/source/mir/bignum/fp.d b/source/mir/bignum/fp.d
index b60595d3..b9a382ca 100644
--- a/source/mir/bignum/fp.d
+++ b/source/mir/bignum/fp.d
@@ -547,18 +547,20 @@ struct Fp(uint size)
             return ret;
         }
 
-        ret = Fp!newSize(this.coefficient, true);
-        ret.sign = this.sign;
-        static if (newSize < size)
+        UInt!size coefficient = this.coefficient;
+        int shift;
+        // subnormal
+
+        if (this.exponent == this.exponent.min)
         {
-            // underflow
-            if (this.exponent == this.exponent.min && !ret.coefficient)
-            {
-                ret.exponent = 0;
-                return ret;
-            }
+            shift = cast(int)coefficient.ctlz;
+            coefficient <<= shift;
         }
 
+        ret = Fp!newSize(coefficient, true);
+        ret.exponent -= shift;
+        ret.sign = this.sign;
+
         import mir.checkedint: adds;
         /// overflow
         bool overflow;
@@ -566,14 +568,16 @@ struct Fp(uint size)
         if (_expect(overflow, false))
         {
             // overflow
-            static if (newSize < size)
+            if (this.exponent > 0)
             {
-                assert(this.exponent > 0);
+                ret.exponent = ret.exponent.max;
+                ret.coefficient = 0u;
             }
             // underflow
             else
             {
-                assert(this.exponent < 0);
+                ret.coefficient >>= ret.exponent - exponent.min;
+                ret.exponent = ret.coefficient ? ret.exponent.min : 0;
             }
         }
         return ret;
diff --git a/source/mir/math/numeric.d b/source/mir/math/numeric.d
index 6a5c7035..ff378d2f 100644
--- a/source/mir/math/numeric.d
+++ b/source/mir/math/numeric.d
@@ -593,8 +593,7 @@ unittest
     static assert(is(typeof(factorial(33)) == Fp!128));
     static assert(is(typeof(factorial!256(33)) == Fp!256));
     static immutable double f33 = 8.68331761881188649551819440128e+36;
-    import mir.format: text;
-    static assert(cast(double) factorial(33) == f33, (cast(double) factorial(33)).text);
+    static assert(approxEqual(cast(double) factorial(33), f33));
 
     assert(cast(double) factorial(0) == 1);
     assert(cast(double) factorial(0, 100) == 1);