diff --git a/api.go b/api.go
index f984b1c49f..9bba627743 100644
--- a/api.go
+++ b/api.go
@@ -892,8 +892,7 @@ func postBuild(srv *Server, version float64, w http.ResponseWriter, r *http.Requ
 	b := NewBuildFile(srv, utils.NewWriteFlusher(w), !suppressOutput, !noCache, rm)
 	id, err := b.Build(context)
 	if err != nil {
-		fmt.Fprintf(w, "Error build: %s\n", err)
-		return err
+		return fmt.Errorf("Error build: %s", err)
 	}
 	if repoName != "" {
 		srv.runtime.repositories.Set(repoName, tag, id, false)
diff --git a/runtime.go b/runtime.go
index aff1773fdf..cb37a82304 100644
--- a/runtime.go
+++ b/runtime.go
@@ -282,7 +282,9 @@ func (runtime *Runtime) Create(config *Config) (*Container, error) {
 	}
 
 	if img.Config != nil {
-		MergeConfig(config, img.Config)
+		if err := MergeConfig(config, img.Config); err != nil {
+			return nil, err
+		}
 	}
 
 	if len(config.Entrypoint) != 0 && config.Cmd == nil {
diff --git a/utils.go b/utils.go
index aed8ffdd76..769f49919a 100644
--- a/utils.go
+++ b/utils.go
@@ -65,7 +65,7 @@ func CompareConfig(a, b *Config) bool {
 	return true
 }
 
-func MergeConfig(userConf, imageConf *Config) {
+func MergeConfig(userConf, imageConf *Config) error {
 	if userConf.User == "" {
 		userConf.User = imageConf.User
 	}
@@ -85,7 +85,10 @@ func MergeConfig(userConf, imageConf *Config) {
 			found := false
 			imageNat, _ := parseNat(imagePortSpec)
 			for _, userPortSpec := range userConf.PortSpecs {
-				userNat, _ := parseNat(userPortSpec)
+				userNat, err := parseNat(userPortSpec)
+				if err != nil {
+					return err
+				}
 				if imageNat.Proto == userNat.Proto && imageNat.Backend == userNat.Backend {
 					found = true
 				}
@@ -146,6 +149,7 @@ func MergeConfig(userConf, imageConf *Config) {
 			userConf.Volumes[k] = v
 		}
 	}
+	return nil
 }
 
 func parseLxcConfOpts(opts ListOpts) ([]KeyValuePair, error) {