diff --git a/config/raw_config_test.go b/config/raw_config_test.go
index b663f62c47..d912156bec 100644
--- a/config/raw_config_test.go
+++ b/config/raw_config_test.go
@@ -1,3 +1 @@
 package config
-
-
diff --git a/config/variable.go b/config/variable.go
index 4fac2c1223..ce83097cac 100644
--- a/config/variable.go
+++ b/config/variable.go
@@ -99,8 +99,10 @@ func (w *variableDetectWalker) Primitive(v reflect.Value) error {
 // will _panic_. The variableDetectWalker will tell you all variables
 // you need.
 type variableReplaceWalker struct {
-	Values map[string]string
+	Values      map[string]string
+	UnknownKeys []string
 
+	key    []string
 	loc    reflectwalk.Location
 	m, mk  reflect.Value
 	cs     []reflect.Value
@@ -118,6 +120,8 @@ func (w *variableReplaceWalker) Exit(loc reflectwalk.Location) error {
 	switch loc {
 	case reflectwalk.Map:
 		w.cs = w.cs[:len(w.cs)-1]
+	case reflectwalk.MapValue:
+		w.key = w.key[:len(w.key)-1]
 	}
 
 	return nil
@@ -132,6 +136,7 @@ func (w *variableReplaceWalker) MapElem(m, k, v reflect.Value) error {
 	w.m = m
 	w.mk = k
 	w.csData = k
+	w.key = append(w.key, k.String())
 	return nil
 }
 
@@ -202,4 +207,7 @@ func (w *variableReplaceWalker) removeCurrent() {
 		k := w.csData.(reflect.Value)
 		c.SetMapIndex(k, val)
 	}
+
+	// Append the key to the unknown keys
+	w.UnknownKeys = append(w.UnknownKeys, strings.Join(w.key, "."))
 }
diff --git a/config/variable_test.go b/config/variable_test.go
index 1579a8b99b..bd3a6a96e6 100644
--- a/config/variable_test.go
+++ b/config/variable_test.go
@@ -183,3 +183,65 @@ func TestVariableReplaceWalker(t *testing.T) {
 		}
 	}
 }
+
+func TestVariableReplaceWalker_unknown(t *testing.T) {
+	cases := []struct {
+		Input  interface{}
+		Output interface{}
+		Keys   []string
+	}{
+		{
+			map[string]interface{}{
+				"foo": "bar",
+				"bar": "hello${var.unknown}world",
+			},
+			map[string]interface{}{
+				"foo": "bar",
+			},
+			[]string{"bar"},
+		},
+		{
+			map[string]interface{}{
+				"foo": []string{"foo", "${var.unknown}", "bar"},
+			},
+			map[string]interface{}{},
+			[]string{"foo"},
+		},
+		{
+			map[string]interface{}{
+				"foo": map[string]interface{}{
+					"bar": "${var.unknown}",
+				},
+			},
+			map[string]interface{}{
+				"foo": map[string]interface{}{},
+			},
+			[]string{"foo.bar"},
+		},
+	}
+
+	for i, tc := range cases {
+		var input interface{} = tc.Input
+		w := &variableReplaceWalker{
+			Values: map[string]string{
+				"var.unknown": UnknownVariableValue,
+			},
+		}
+
+		if reflect.ValueOf(tc.Input).Kind() == reflect.String {
+			input = &tc.Input
+		}
+
+		if err := reflectwalk.Walk(input, w); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		if !reflect.DeepEqual(tc.Input, tc.Output) {
+			t.Fatalf("bad %d: %#v", i, tc.Input)
+		}
+
+		if !reflect.DeepEqual(tc.Keys, w.UnknownKeys) {
+			t.Fatalf("bad: %#v", w.UnknownKeys)
+		}
+	}
+}