diff --git a/packages/firestore/src/util/sorted_map.ts b/packages/firestore/src/util/sorted_map.ts
index 0c144066ce0..a24cf8802ca 100644
--- a/packages/firestore/src/util/sorted_map.ts
+++ b/packages/firestore/src/util/sorted_map.ts
@@ -197,7 +197,7 @@ export class SortedMapIterator<K, V> {
     while (!node.isEmpty()) {
       cmp = startKey ? comparator(node.key, startKey) : 1;
       // flip the comparison if we're going in reverse
-      if (isReverse) {
+      if (startKey && isReverse) {
         cmp *= -1;
       }
 
diff --git a/packages/firestore/test/unit/util/sorted_map.test.ts b/packages/firestore/test/unit/util/sorted_map.test.ts
index c52ded7403d..7308268e2e9 100644
--- a/packages/firestore/test/unit/util/sorted_map.test.ts
+++ b/packages/firestore/test/unit/util/sorted_map.test.ts
@@ -19,7 +19,11 @@ import { expect } from 'chai';
 
 import { primitiveComparator } from '../../../src/util/misc';
 import { forEach } from '../../../src/util/obj';
-import { LLRBNode, SortedMap } from '../../../src/util/sorted_map';
+import {
+  LLRBNode,
+  SortedMap,
+  SortedMapIterator
+} from '../../../src/util/sorted_map';
 
 function shuffle(arr: number[]): void {
   for (let i = arr.length - 1; i > 0; i--) {
@@ -32,6 +36,41 @@ function shuffle(arr: number[]): void {
 // Many of these were adapted from the mugs source code.
 // http://mads379.github.com/mugs/
 describe('SortedMap Tests', () => {
+  /** Returns a map with (i, i) entries for i in [0, 9] inclusive */
+  function getMapOfNumbersZeroToNine(): SortedMap<number, number> {
+    const items: number[] = [];
+    for (let i = 0; i < 10; i++) {
+      items.push(i);
+    }
+    shuffle(items);
+
+    let map = new SortedMap<number, number>(primitiveComparator);
+    for (let i = 0; i < 10; i++) {
+      map = map.insert(items[i], items[i]);
+    }
+    return map;
+  }
+
+  /**
+   * Validates that the given iterator covers the given range by counting from
+   * the given `from` argument to the given `to` argument (inclusive).
+   */
+  function validateIteratorCoversRange(
+    iterator: SortedMapIterator<number, number>,
+    from: number,
+    to: number
+  ): void {
+    const countUp = from <= to;
+    let expected = from;
+    while (iterator.hasNext()) {
+      const n = iterator.getNext();
+      expect(n.key).to.equal(expected);
+      expect(n.value).to.equal(expected);
+      expected = countUp ? expected + 1 : expected - 1;
+    }
+    expect(expected).to.equal(countUp ? to + 1 : to - 1);
+  }
+
   it('Create node', () => {
     const map = new SortedMap(primitiveComparator).insert('key', 'value');
     expect(map.root.left.isEmpty()).to.equal(true);
@@ -405,27 +444,46 @@ describe('SortedMap Tests', () => {
     expect(() => iterator.getNext()).to.throw();
   });
 
-  it('SortedMapIterator test with 10 items.', () => {
-    const items: number[] = [];
-    for (let i = 0; i < 10; i++) {
-      items.push(i);
-    }
-    shuffle(items);
+  it('forward iterator without start key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getIterator();
+    validateIteratorCoversRange(iterator, 0, 9);
+  });
 
-    let map = new SortedMap(primitiveComparator);
-    for (let i = 0; i < 10; i++) {
-      map = map.insert(items[i], items[i]);
-    }
+  it('forward iterator with start key.', () => {
+    const iterator = getMapOfNumbersZeroToNine().getIteratorFrom(5);
+    validateIteratorCoversRange(iterator, 5, 9);
+  });
 
-    const iterator = map.getIterator();
-    let expected = 0;
-    while (iterator.hasNext()) {
-      const n = iterator.getNext();
-      expect(n.key).to.equal(expected);
-      expect(n.value).to.equal(expected);
-      expected++;
-    }
-    expect(expected).to.equal(10);
+  it('forward iterator with start key larger than max key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getIteratorFrom(50);
+    expect(iterator.hasNext()).to.equal(false);
+    expect(() => iterator.getNext()).to.throw();
+  });
+
+  it('forward iterator with start key smaller than min key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getIteratorFrom(-50);
+    validateIteratorCoversRange(iterator, 0, 9);
+  });
+
+  it('reverse iterator without start key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getReverseIterator();
+    validateIteratorCoversRange(iterator, 9, 0);
+  });
+
+  it('reverse iterator with start key.', () => {
+    const iterator = getMapOfNumbersZeroToNine().getReverseIteratorFrom(7);
+    validateIteratorCoversRange(iterator, 7, 0);
+  });
+
+  it('reverse iterator with start key smaller than min key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getReverseIteratorFrom(-50);
+    expect(iterator.hasNext()).to.equal(false);
+    expect(() => iterator.getNext()).to.throw();
+  });
+
+  it('reverse iterator with start key larger than max key', () => {
+    const iterator = getMapOfNumbersZeroToNine().getReverseIteratorFrom(50);
+    validateIteratorCoversRange(iterator, 9, 0);
   });
 
   it('SortedMap.indexOf returns index.', () => {