Index: subversion/tests/cmdline/merge_tests.py
===================================================================
--- subversion/tests/cmdline/merge_tests.py	(revision 32959)
+++ subversion/tests/cmdline/merge_tests.py	(working copy)
@@ -12636,6 +12636,37 @@
   # Surprise: if we don't merge the file's 'rev1' state first, it doesn't fail
   # nor even raise a conflict.
 
+def merge_file_with_space_in_its_path(sbox):
+  "merge a file with space in its path"
+
+  sbox.build()
+  wc_dir = sbox.wc_dir
+  some_dir = os.path.join(wc_dir, "some dir")
+  file1 = os.path.join(some_dir, "file1")
+  file2 = os.path.join(some_dir, "file2")
+
+  # Make r2.
+  os.mkdir(some_dir)
+  svntest.main.file_append(file1, "Initial text in the file.\n")
+  svntest.main.run_svn(None, "add", some_dir)
+  svntest.actions.run_and_verify_svn(None, None, [],
+                                     "ci", "-m", "r2", wc_dir)
+
+  # Make r3.
+  svntest.main.run_svn(None, "copy", file1, file2)
+  svntest.actions.run_and_verify_svn(None, None, [],
+                                     "ci", "-m", "r3", wc_dir)
+
+  # Make r4.
+  svntest.main.file_append(file2, "Next line of text in the file.\n")
+  svntest.actions.run_and_verify_svn(None, None, [],
+                                     "ci", "-m", "r4", wc_dir)
+
+  target_url = sbox.repo_url + '/some%20dir/file2'
+  svntest.actions.run_and_verify_svn(None, None, [],
+                                     "merge", "--reintegrate", target_url,
+                                     file1)
+
 ########################################################################
 # Run the tests
 
@@ -12819,6 +12850,7 @@
                          server_has_mergeinfo),
               XFail(merge_two_edits_to_same_prop),
               merge_an_eol_unification_and_set_svn_eol_style,
+              merge_file_with_space_in_its_path,
              ]
 
 if __name__ == '__main__':
Index: subversion/libsvn_client/merge.c
===================================================================
--- subversion/libsvn_client/merge.c	(revision 32959)
+++ subversion/libsvn_client/merge.c	(working copy)
@@ -6309,10 +6309,12 @@
          over and over. */
       /* We never merged to the source.  Just return the branch point. */
       const char *yc_ancestor_path,
-        *source_url = svn_path_join(source_repos_root, source_repos_rel_path,
-                                    subpool),
-        *target_url = svn_path_join(source_repos_root, target_repos_rel_path,
-                                    subpool);
+        *source_url = svn_path_url_add_component(source_repos_root,
+                                                 source_repos_rel_path,
+                                                 subpool),
+        *target_url = svn_path_url_add_component(source_repos_root,
+                                                 target_repos_rel_path,
+                                                 subpool);
 
       SVN_ERR(svn_client__get_youngest_common_ancestor(&yc_ancestor_path,
                                                        rev_left,
@@ -6324,7 +6326,8 @@
                                  _("'%s@%ld' must be ancestrally related to "
                                    "'%s@%ld'"), source_url, source_rev,
                                  target_url, target_rev);
-      *url_left = svn_path_join(source_repos_root, yc_ancestor_path, pool);
+      *url_left = svn_path_url_add_component(source_repos_root,
+                                             yc_ancestor_path, pool);
       *source_mergeinfo_p = apr_hash_make(pool);
 
       svn_pool_destroy(subpool);