diff --git a/config/expr_lex.go b/config/expr_lex.go
index 9c9d45e16..f94e947ba 100644
--- a/config/expr_lex.go
+++ b/config/expr_lex.go
@@ -14,9 +14,9 @@ const lexEOF = 0
 // The parser uses the type <prefix>Lex as a lexer.  It must provide
 // the methods Lex(*<prefix>SymType) int and Error(string).
 type exprLex struct {
-	Err error
+	Err   error
+	Input string
 
-	input string
 	pos   int
 	width int
 }
@@ -105,12 +105,12 @@ func (x *exprLex) lexString(yylval *exprSymType) int {
 
 // Return the next rune for the lexer.
 func (x *exprLex) next() rune {
-	if int(x.pos) >= len(x.input) {
+	if int(x.pos) >= len(x.Input) {
 		x.width = 0
 		return lexEOF
 	}
 
-	r, w := utf8.DecodeRuneInString(x.input[x.pos:])
+	r, w := utf8.DecodeRuneInString(x.Input[x.pos:])
 	x.width = w
 	x.pos += x.width
 	return r
diff --git a/config/expr_lex_test.go b/config/expr_lex_test.go
new file mode 100644
index 000000000..8e94d86f3
--- /dev/null
+++ b/config/expr_lex_test.go
@@ -0,0 +1,51 @@
+package config
+
+import (
+	"io/ioutil"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestLex(t *testing.T) {
+	cases := []struct {
+		Input  string
+		Output []int
+	}{
+		{
+			"concat.hcl",
+			[]int{IDENTIFIER, LEFTPAREN,
+				STRING, COMMA, STRING, COMMA, STRING,
+				RIGHTPAREN, lexEOF},
+		},
+	}
+
+	for _, tc := range cases {
+		d, err := ioutil.ReadFile(filepath.Join(
+			fixtureDir, "interpolations", tc.Input))
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		l := &exprLex{Input: string(d)}
+		var actual []int
+		for {
+			token := l.Lex(new(exprSymType))
+			actual = append(actual, token)
+
+			if token == lexEOF {
+				break
+			}
+
+			if len(actual) > 500 {
+				t.Fatalf("Input:%s\n\nExausted.", tc.Input)
+			}
+		}
+
+		if !reflect.DeepEqual(actual, tc.Output) {
+			t.Fatalf(
+				"Input: %s\n\nBad: %#v\n\nExpected: %#v",
+				tc.Input, actual, tc.Output)
+		}
+	}
+}
diff --git a/config/expr_parse.go b/config/expr_parse.go
index bfb81c6fb..c6fb39f87 100644
--- a/config/expr_parse.go
+++ b/config/expr_parse.go
@@ -21,7 +21,7 @@ func ExprParse(v string) (Interpolation, error) {
 	exprResult = nil
 
 	// Parse
-	lex := &exprLex{input: v}
+	lex := &exprLex{Input: v}
 	exprParse(lex)
 
 	// Build up the errors
diff --git a/config/test-fixtures/interpolations/concat.hcl b/config/test-fixtures/interpolations/concat.hcl
new file mode 100644
index 000000000..aab6e50aa
--- /dev/null
+++ b/config/test-fixtures/interpolations/concat.hcl
@@ -0,0 +1 @@
+concat("foo","-","0.0/16")