diff --git a/packages/babel-plugin-minify-simplify/__tests__/simplify-test.js b/packages/babel-plugin-minify-simplify/__tests__/simplify-test.js
index 9642c602a..9f15e450c 100644
--- a/packages/babel-plugin-minify-simplify/__tests__/simplify-test.js
+++ b/packages/babel-plugin-minify-simplify/__tests__/simplify-test.js
@@ -2532,4 +2532,29 @@ describe("simplify-plugin", () => {
       }
     `
   );
+
+  // TODO: issue-637
+  thePlugin(
+    "should NOT remove else block when ",
+    `
+      function test(a) {
+        const clash = () => {};
+        if (a) {
+          return clash();
+        } else {
+          const clash = () => {};
+          return clash();
+        }
+      }
+    `,
+    `
+      function test(a) {
+        const clash = () => {};
+        if (a) return clash();else {
+          const clash = () => {};
+          return clash();
+        }
+      }
+    `
+  );
 });
diff --git a/packages/babel-plugin-minify-simplify/src/if-statement.js b/packages/babel-plugin-minify-simplify/src/if-statement.js
index 37cefc1af..61dd213a6 100644
--- a/packages/babel-plugin-minify-simplify/src/if-statement.js
+++ b/packages/babel-plugin-minify-simplify/src/if-statement.js
@@ -276,7 +276,18 @@ module.exports = t => {
         (consequent.isBlockStatement() &&
           t.isReturnStatement(
             consequent.node.body[consequent.node.body.length - 1]
-          )))
+          ))) &&
+      // don't hoist declarations
+      // TODO: validate declarations after fixing scope issues
+      (alternate.isBlockStatement()
+        ? !alternate
+            .get("body")
+            .some(
+              stmt =>
+                stmt.isVariableDeclaration({ kind: "let" }) ||
+                stmt.isVariableDeclaration({ kind: "const" })
+            )
+        : true)
     ) {
       path.insertAfter(
         alternate.isBlockStatement()