diff --git a/config/variable.go b/config/variable.go
index 0fd0ebd841..841981a39b 100644
--- a/config/variable.go
+++ b/config/variable.go
@@ -16,14 +16,18 @@ func init() {
 	varRegexp = regexp.MustCompile(`(?i)(\$+)\{([-.a-z0-9_]+)\}`)
 }
 
-// replaceVariables takes a configuration and a mapping of variables
+// ReplaceVariables takes a configuration and a mapping of variables
 // and performs the structure walking necessary to properly replace
 // all the variables.
-func replaceVariables(
-	c map[string]interface{},
-	vs map[string]string) error {
+func ReplaceVariables(
+	c interface{},
+	vs map[string]string) ([]string, error) {
 	w := &variableReplaceWalker{Values: vs}
-	return reflectwalk.Walk(c, w)
+	if err := reflectwalk.Walk(c, w); err != nil {
+		return nil, err
+	}
+
+	return w.UnknownKeys, nil
 }
 
 // variableDetectWalker implements interfaces for the reflectwalk package
diff --git a/config/variable_test.go b/config/variable_test.go
index bd3a6a96e6..88fdb23e10 100644
--- a/config/variable_test.go
+++ b/config/variable_test.go
@@ -36,6 +36,25 @@ func BenchmarkVariableReplaceWalker(b *testing.B) {
 	}
 }
 
+func TestReplaceVariables(t *testing.T) {
+	input := "foo-${var.bar}"
+	expected := "foo-bar"
+
+	unk, err := ReplaceVariables(&input, map[string]string{
+		"var.bar": "bar",
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if len(unk) > 0 {
+		t.Fatal("bad: %#v", unk)
+	}
+
+	if input != expected {
+		t.Fatalf("bad: %#v", input)
+	}
+}
+
 func TestVariableDetectWalker(t *testing.T) {
 	w := new(variableDetectWalker)