error out when a referenced provider is missing

terraform/test-fixtures/transform-provider-invalid/main.tf

@@ -0,0 +1,11 @@
+provider "aws" {
+}
+
+module "mod" {
+  source = "./mod"
+  
+  # aws.foo doesn't exist, and should report an error
+  providers = {
+    "aws" = "aws.foo"
+  }
+}

terraform/test-fixtures/transform-provider-invalid/mod/main.tf

@@ -0,0 +1,2 @@
+resource "aws_resource" "foo" {
+}

terraform/transform_provider.go

@@ -428,7 +428,7 @@ func (t *ProviderConfigTransformer) Transform(g *Graph) error {
 
 	// Start the transformation process
 	if err := t.transform(g, t.Module); err != nil {
-		return nil
+		return err
 	}
 
 	// finally attach the configs to the new nodes
@@ -524,6 +524,7 @@ func (t *ProviderConfigTransformer) addProxyProviders(g *Graph, m *module.Tree)
 	for name, parentName := range parentCfg.Providers {
 		fullName := ResolveProviderName(name, path)
 		fullParentName := ResolveProviderName(parentName, parentPath)
+
 		parentProvider := t.providers[fullParentName]
 
 		if parentProvider == nil {

terraform/transform_provider_test.go

@@ -545,6 +545,35 @@ provider.aws.foo`)
 	}
 }
 
+// error out when a non-existent provider is named in a module providers map
+func TestProviderConfigTransformer_invalidProvider(t *testing.T) {
+	mod := testModule(t, "transform-provider-invalid")
+	concrete := func(a *NodeAbstractProvider) dag.Vertex { return a }
+
+	g := Graph{Path: RootModulePath}
+	{
+		tf := &ConfigTransformer{Module: mod}
+		if err := tf.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+	{
+		tf := &AttachResourceConfigTransformer{Module: mod}
+		if err := tf.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	tf := TransformProviders([]string{"aws"}, concrete, mod)
+	err := tf.Transform(&g)
+	if err == nil {
+		t.Fatal("expected missing provider error")
+	}
+	if !strings.Contains(err.Error(), "provider.aws.foo") {
+		t.Fatalf("error should reference missing provider, got: %s", err)
+	}
+}
+
 const testTransformProviderBasicStr = `
 aws_instance.web
   provider.aws