diff --git a/config/variable.go b/config/variable.go
index edf8b671c8..e26fa85bb6 100644
--- a/config/variable.go
+++ b/config/variable.go
@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"reflect"
 	"regexp"
 	"strings"
@@ -53,6 +54,13 @@ func (w *variableDetectWalker) Primitive(v reflect.Value) error {
 
 		var err error
 		var iv InterpolatedVariable
+		if strings.Index(key, ".") == -1 {
+			return fmt.Errorf(
+				"Interpolated variable '%s' has bad format. "+
+					"Did you mean 'var.%s'?",
+				key, key)
+		}
+
 		if strings.HasPrefix(key, "var.") {
 			iv, err = NewUserVariable(key)
 		} else {
diff --git a/config/variable_test.go b/config/variable_test.go
new file mode 100644
index 0000000000..bef0f60af7
--- /dev/null
+++ b/config/variable_test.go
@@ -0,0 +1,58 @@
+package config
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestVariableDetectWalker(t *testing.T) {
+	w := new(variableDetectWalker)
+
+	str := `foo ${var.bar}`
+	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if len(w.Variables) != 1 {
+		t.Fatalf("bad: %#v", w.Variables)
+	}
+	if w.Variables["var.bar"].(*UserVariable).FullKey() != "var.bar" {
+		t.Fatalf("bad: %#v", w.Variables)
+	}
+}
+
+func TestVariableDetectWalker_bad(t *testing.T) {
+	w := new(variableDetectWalker)
+
+	str := `foo ${bar}`
+	if err := w.Primitive(reflect.ValueOf(str)); err == nil {
+		t.Fatal("should error")
+	}
+}
+
+func TestVariableDetectWalker_escaped(t *testing.T) {
+	w := new(variableDetectWalker)
+
+	str := `foo $${var.bar}`
+	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if len(w.Variables) > 0 {
+		t.Fatalf("bad: %#v", w.Variables)
+	}
+}
+
+func TestVariableDetectWalker_empty(t *testing.T) {
+	w := new(variableDetectWalker)
+
+	str := `foo`
+	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if len(w.Variables) > 0 {
+		t.Fatalf("bad: %#v", w.Variables)
+	}
+}
+