diff --git a/proto/decode.go b/proto/decode.go
index 5810782..f94b9f4 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -768,10 +768,11 @@
 		}
 	}
 	keyelem, valelem := keyptr.Elem(), valptr.Elem()
-	if !keyelem.IsValid() || !valelem.IsValid() {
-		// We did not decode the key or the value in the map entry.
-		// Either way, it's an invalid map entry.
-		return fmt.Errorf("proto: bad map data: missing key/val")
+	if !keyelem.IsValid() {
+		keyelem = reflect.Zero(p.mtype.Key())
+	}
+	if !valelem.IsValid() {
+		valelem = reflect.Zero(p.mtype.Elem())
 	}
 
 	v.SetMapIndex(keyelem, valelem)