diff --git a/src/crypto/dsa/dsa.go b/src/crypto/dsa/dsa.go
index 575314b..2fc4f1f 100644
--- a/src/crypto/dsa/dsa.go
+++ b/src/crypto/dsa/dsa.go
@@ -279,6 +279,9 @@
 	}
 
 	w := new(big.Int).ModInverse(s, pub.Q)
+	if w == nil {
+		return false
+	}
 
 	n := pub.Q.BitLen()
 	if n&7 != 0 {