diff --git a/src/math/big/rat.go b/src/math/big/rat.go
index c7f79a5666..3b66507966 100644
--- a/src/math/big/rat.go
+++ b/src/math/big/rat.go
@@ -162,7 +162,7 @@ func quotToFloat32(stk *stack, a, b nat) (f float32, exact bool) {
 	mantissa >>= 1 // discard rounding bit.  Mantissa now scaled by 1<<Msize1.
 
 	f = float32(math.Ldexp(float64(mantissa), exp-Msize1))
-	if math.IsInf(float64(f), 0) {
+	if math.IsInf(float64(f), 0) || f == 0 {
 		exact = false
 	}
 	return
@@ -260,7 +260,7 @@ func quotToFloat64(stk *stack, a, b nat) (f float64, exact bool) {
 	mantissa >>= 1 // discard rounding bit.  Mantissa now scaled by 1<<Msize1.
 
 	f = math.Ldexp(float64(mantissa), exp-Msize1)
-	if math.IsInf(f, 0) {
+	if math.IsInf(f, 0) || f == 0 {
 		exact = false
 	}
 	return
diff --git a/src/math/big/ratconv_test.go b/src/math/big/ratconv_test.go
index 93e89ad1c8..4359b741a4 100644
--- a/src/math/big/ratconv_test.go
+++ b/src/math/big/ratconv_test.go
@@ -466,6 +466,11 @@ var float64inputs = []string{
 	"-1152921504606846977", // -(1<<60 + 1)
 
 	"1/3",
+
+	"170141183460469231731687303715884105728p-48317",
+	"0000000000000000000000000000031768211457p-9818716",
+	"00000000000000000000000000000000000000000000000000000000001662320182p-801401",
+	"818p-8818873",
 }
 
 // isFinite reports whether f represents a finite rational value.