diff --git a/src/lib/expr.test.ts b/src/lib/expr.test.ts
index 31b4714..a4917c9 100644
--- a/src/lib/expr.test.ts
+++ b/src/lib/expr.test.ts
@@ -2,21 +2,29 @@ import { AsText, TextPiece } from "../words";
 import { Expr } from "./expr";
 
 describe("expr", () => {
-  test.each([
-    ["1", "1"],
-    ["1 + 2", "3"],
-    ["1 - 2", "-1"],
-    ["1 * 2", "2"],
-    ["1 / 2", "0.5"],
-    ["1 // 2", "0"],
-    // TODO: operator precedence
-    // TODO: parentheses
-    // TODO; eror reporting
-  ])("Expr does math: %s", (expression, result) => {
-    const actualResult = Expr({}, [{ text: expression }]);
-    console.log(actualResult)
-    expect("error" in actualResult).toBeFalsy();
-    expect(AsText(actualResult as TextPiece)).toEqual(result);
+  describe("Expr does math", () => {
+    test.each([
+      ["1", "1"],
+      ["1 + 2", "3"],
+      ["1 - 2", "-1"],
+      ["1 * 2", "2"],
+      ["1 / 2", "0.5"],
+      ["1 // 2", "0"],
+      // TODO: operator precedence
+      // TODO: parentheses
+    ])("%s", (expression, result) => {
+      const actualResult = Expr({}, [{ text: expression }]);
+      expect("error" in actualResult).toBeFalsy();
+      expect(AsText(actualResult as TextPiece)).toEqual(result);
+    });
+  });
+
+  // TODO; error reporting
+  describe("Expr rejects invalid expressions", () => {
+    test.each([["1 $ 2"], ["1 1 + 2"], ["$ 1"]])("%s", (expression) => {
+      const actualResult = Expr({}, [{ text: expression }]);
+      expect("error" in actualResult).toBeTruthy();
+    });
   });
 
   // TODO: handle expr prefix
diff --git a/src/lib/expr.ts b/src/lib/expr.ts
index 880ee2a..f4a72de 100644
--- a/src/lib/expr.ts
+++ b/src/lib/expr.ts
@@ -12,6 +12,10 @@ export function Expr({}, argv: TextPiece[]): ProcResult {
   const parser = new ExpressionParser(expression);
   const result = parser.parseSubExpression(0);
 
+  if(parser.pos != expression.length) {
+    return {error: "Couldn't parse full expression"}
+  }
+
   if ("value" in result) {
     return { text: String(result.value) };
   } else {