diff --git a/lib/techs/mock-lang-js.js b/lib/techs/mock-lang-js.js
index fb33601..6bc91ce 100644
--- a/lib/techs/mock-lang-js.js
+++ b/lib/techs/mock-lang-js.js
@@ -8,19 +8,30 @@ module.exports = require('enb/lib/build-flow').create()
         return vfs.read(source, 'utf8')
             .then(function (content) {
                 var mock = [
-                    '(function(global, bem_) {',
-                    '    if(bem_.I18N) return;',
-                    '    global.BEM = bem_;',
-                    '    var i18n = bem_.I18N = function(keyset, key) {',
-                    '        return key;',
-                    '    };',
-                    '    i18n.keyset = function() { return i18n }',
-                    '    i18n.key = function(key) { return key }',
-                    '    i18n.lang = function() { return }',
-                    '})(this, typeof BEM === \'undefined\' ? {} : BEM);'
-                ].join('\n');
+                        '(function(global, bem_) {',
+                        '    if(bem_.I18N) return;',
+                        '    global.BEM = bem_;',
+                        '    var i18n = bem_.I18N = function(keyset, key) {',
+                        '        return key;',
+                        '    };',
+                        '    i18n.keyset = function() { return i18n }',
+                        '    i18n.key = function(key) { return key }',
+                        '    i18n.lang = function() { return }',
+                        '})(this, typeof BEM === \'undefined\' ? {} : BEM);'
+                    ].join('\n'),
+                    mapIndex = content.lastIndexOf('//# sourceMappingURL='),
+                    map;
 
-                return [mock, content].join('\n');
+                // if there is a #sourceMappingURL pragma append
+                // the mock before it so the source map will be
+                // valid. We can't insert it in the beginning because
+                // source map locations will point to the wrong lines.
+                if (mapIndex !== -1) {
+                    map = content.substring(mapIndex);
+                    content = content.substring(0, mapIndex);
+                }
+
+                return [content, mock, map].join('\n');
             });
     })
     .createTech();