diff --git a/CHANGELOG.md b/CHANGELOG.md
index 99619f68f..e60ec4707 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprocket
 ## Master
 
 - Fix for Ruby 2.7 keyword arguments warning in `base.rb`. [#660](https://github.com/rails/sprockets/pull/660)
+- Fix for when `x_sprockets_linecount` is missing from a source map.
 - Remove remaining support for Ruby < 2.4.[#672](https://github.com/rails/sprockets/pull/672)
 
 ## 4.0.0
diff --git a/UPGRADING.md b/UPGRADING.md
index 37b221516..dcab46401 100644
--- a/UPGRADING.md
+++ b/UPGRADING.md
@@ -53,7 +53,7 @@ If you wanted to specify additional assets to deliver that were not included by
 config.assets.precompile += ["marketing.css"]
 ```
 
-If you are using Sprockets 4, Rails changes it's default logic for determining top-level targets.  It will now use _only_ a file at `./app/assets/config/manifest.js` for specifying top-level targets; this file may already exist in your Rails app (although Rails only starts automatically using it once you are using sprockets 4), if not you should create it.
+If you are using Sprockets 4, Rails changes its default logic for determining top-level targets.  It will now use _only_ a file at `./app/assets/config/manifest.js` for specifying top-level targets; this file may already exist in your Rails app (although Rails only starts automatically using it once you are using sprockets 4), if not you should create it.
 
 The `manifest.js` file is meant to specify which files to use as a top-level target using sprockets methods `link`, `link_directory`, and `link_tree`.
 
diff --git a/lib/sprockets/source_map_utils.rb b/lib/sprockets/source_map_utils.rb
index f1b4cce88..b686bbb50 100644
--- a/lib/sprockets/source_map_utils.rb
+++ b/lib/sprockets/source_map_utils.rb
@@ -78,7 +78,7 @@ def concat_source_maps(a, b)
       offset = 0
       if a["sections"].count != 0 && !a["sections"].last["map"]["mappings"].empty?
         last_line_count = a["sections"].last["map"].delete("x_sprockets_linecount")
-        offset += last_line_count
+        offset += last_line_count || 1
 
         last_offset = a["sections"].last["offset"]["line"]
         offset += last_offset
diff --git a/test/test_source_map_utils.rb b/test/test_source_map_utils.rb
index febca7b45..1fff15d7a 100644
--- a/test/test_source_map_utils.rb
+++ b/test/test_source_map_utils.rb
@@ -177,6 +177,51 @@ def test_concat_source_maps
     ], Sprockets::SourceMapUtils.decode_source_map(combined)[:mappings]
   end
 
+  def test_concat_without_x_sprockets_linecount_works
+    abc_mappings = [
+      { source: 'a.js', generated: [1, 0], original: [1,  0] },
+      { source: 'b.js', generated: [2, 0], original: [20, 0] },
+      { source: 'c.js', generated: [3, 0], original: [30, 0] }
+    ].freeze
+
+    d_mapping = [
+      { source: 'd.js', generated: [1, 0], original: [1, 0] }
+    ].freeze
+
+    abc_map = {
+      "version" => 3,
+      "file" => "a.js",
+      "mappings" => Sprockets::SourceMapUtils.encode_vlq_mappings(abc_mappings),
+      "sources" => ["a.js", "b.js", "c.js"],
+      "names" => [],
+      "x_sprockets_linecount" => 3
+    }
+
+    d_map = {
+      "version" => 3,
+      "file" => "d.js",
+      "mappings" => Sprockets::SourceMapUtils.encode_vlq_mappings(d_mapping),
+      "sources" => ["d.js"],
+      "names" => [],
+    }
+
+    combined = Sprockets::SourceMapUtils.concat_source_maps(deep_dup(abc_map), deep_dup(d_map))
+    assert_equal [
+      { source: 'a.js', generated: [1, 0], original: [1,  0] },
+      { source: 'b.js', generated: [2, 0], original: [20, 0] },
+      { source: 'c.js', generated: [3, 0], original: [30, 0] },
+      { source: 'd.js', generated: [4, 0], original: [1,  0] }
+    ], Sprockets::SourceMapUtils.decode_source_map(combined)[:mappings]
+
+    combined = Sprockets::SourceMapUtils.concat_source_maps(deep_dup(d_map), deep_dup(abc_map))
+    assert_equal [
+      { source: 'd.js', generated: [1, 0], original: [1,  0] },
+      { source: 'a.js', generated: [2, 0], original: [1,  0] },
+      { source: 'b.js', generated: [3, 0], original: [20, 0] },
+      { source: 'c.js', generated: [4, 0], original: [30, 0] }
+    ], Sprockets::SourceMapUtils.decode_source_map(combined)[:mappings]
+  end
+
   def test_combine_source_maps_returns_original
     abc_mappings = [
       { source: 'a.js', generated: [1, 0], original: [0,  0] },