diff --git a/lib/propshaft/compiler/source_mapping_urls.rb b/lib/propshaft/compiler/source_mapping_urls.rb
index 30e57d5..bad3f0e 100644
--- a/lib/propshaft/compiler/source_mapping_urls.rb
+++ b/lib/propshaft/compiler/source_mapping_urls.rb
@@ -11,7 +11,7 @@ def compile(logical_path, input)
 
   private
     def asset_path(source_mapping_url, logical_path)
-      source_mapping_url.gsub!(/^\/?#{url_prefix}\//, "")
+      source_mapping_url.gsub!(/^(.+\/)?#{url_prefix}\//, "")
 
       if logical_path.dirname.to_s == "."
         source_mapping_url
diff --git a/test/fixtures/assets/mapped/sourceMappingURL-already-prefixed-invalid.js b/test/fixtures/assets/mapped/sourceMappingURL-already-prefixed-invalid.js
new file mode 100644
index 0000000..a52b8dd
--- /dev/null
+++ b/test/fixtures/assets/mapped/sourceMappingURL-already-prefixed-invalid.js
@@ -0,0 +1 @@
+var fun; //# sourceMappingURL=thisisinvalidassets/sourceMappingURL-already-prefixed-invalid.js.map
diff --git a/test/fixtures/assets/mapped/sourceMappingURL-already-prefixed-invalid.js.map b/test/fixtures/assets/mapped/sourceMappingURL-already-prefixed-invalid.js.map
new file mode 100644
index 0000000..e69de29
diff --git a/test/propshaft/compiler/source_mapping_urls_test.rb b/test/propshaft/compiler/source_mapping_urls_test.rb
index 5decc09..05a4d7b 100644
--- a/test/propshaft/compiler/source_mapping_urls_test.rb
+++ b/test/propshaft/compiler/source_mapping_urls_test.rb
@@ -60,6 +60,11 @@ class Propshaft::Compiler::SourceMappingUrlsTest < ActiveSupport::TestCase
                  compile_asset(find_asset("nested/sourceMappingURL-already-prefixed-nested.js", fixture_path: "mapped"))
   end
 
+  test "sourceMapURL is already prefixed with an incorrect url_prefix" do
+    refute_match %r{//# sourceMappingURL=thisisinvalidassets/sourceMappingURL-already-prefixed-invalid.js-[a-z0-9]{40}\.map},
+                 compile_asset(find_asset("sourceMappingURL-already-prefixed-invalid.js", fixture_path: "mapped"))
+  end
+
   test "relative url root" do
     @options.relative_url_root = "/url-root"