config: fix error message in validation

config/config.go

@@ -213,8 +213,8 @@ func (c *Config) Validate() error {
 // are valid in the Validate step.
 func (c *Config) allVariables() map[string][]InterpolatedVariable {
 	result := make(map[string][]InterpolatedVariable)
-	for n, pc := range c.ProviderConfigs {
-		source := fmt.Sprintf("provider config '%s'", n)
+	for _, pc := range c.ProviderConfigs {
+		source := fmt.Sprintf("provider config '%s'", pc.Name)
 		for _, v := range pc.RawConfig.Variables {
 			result[source] = append(result[source], v)
 		}

config/test-fixtures/validate-good/main.tf

@@ -3,6 +3,12 @@ variable "foo" {
     description = "bar";
 }
 
+variable "amis" {
+    default = {
+        "east": "foo",
+    }
+}
+
 provider "aws" {
   access_key = "foo";
   secret_key = "bar";
@@ -16,7 +22,7 @@ resource "aws_security_group" "firewall" {
 }
 
 resource aws_instance "web" {
-    ami = "${var.foo}"
+    ami = "${var.amis.east}"
     security_groups = [
         "foo",
         "${aws_security_group.firewall.foo}"