Index: contrib/client-side/svnmerge.py
===================================================================
--- contrib/client-side/svnmerge.py	(revision 19186)
+++ contrib/client-side/svnmerge.py	(working copy)
@@ -630,6 +630,51 @@
     return os.path.isdir(os.path.join(dir, ".svn")) or \
            os.path.isdir(os.path.join(dir, "_svn"))
 
+def wc_of_same_repos(branch_dir, head):
+    """Check if the branch_dir and head dir belong
+    to the same repository.
+    Return False, even if either of the provided dirs
+    is not a working copy."""
+
+    # read the branch_dir entries file
+    if os.path.isdir(os.path.join(branch_dir, ".svn")):
+       _svn_dir = ".svn"
+    elif os.path.isdir(os.path.join(branch_dir, "_svn")):
+       _svn_dir = "_svn"
+    else:
+       # is not a working copy
+       return False
+
+    branch_dir_entries = open(os.path.join( \
+                         branch_dir,_svn_dir,"entries"),"r").readlines()
+
+    # read the head dir entries file
+    if os.path.isdir(os.path.join(head, ".svn")):
+       _svn_dir = ".svn"
+    elif os.path.isdir(os.path.join(head, "_svn")):
+       _svn_dir = "_svn"
+    else:
+       # is not a working copy
+       return False
+
+    head_entries = open(os.path.join( \
+                        head,_svn_dir,"entries"),"r").readlines()
+
+    # join list of lines into one searchable line
+    branch_dir_entries =  "".join(branch_dir_entries)
+    head_entries =  "".join(head_entries)
+
+    # extract the value of the 'repos' key
+    repos_re = re.compile(".*repos\=\"(.*?)\".*", re.DOTALL)
+    branch_repos = repos_re.findall(branch_dir_entries)[0]
+    head_repos = repos_re.findall(head_entries)[0]
+ 
+    # return True if the repos keys are identical
+    if branch_repos == head_repos:
+      return True
+
+    return False
+
 _cache_svninfo = {}
 def get_svninfo(path):
     """Extract the subversion information for a path (through 'svn info').
@@ -1679,6 +1724,11 @@
         head = rstrip(head, "/")
         if not is_wc(head) and not is_url(head):
             error('"%s" is not a valid URL or working directory' % head)
+        # check to see if the head specific belongs to the same repos
+        if is_wc(head) and not wc_of_same_repos(branch_dir, head):
+          err_msg  = '"%s" and "%s"' % (head, branch_dir)
+          err_msg += 'are working copies of different repositories'
+          error(err_msg)
         opts["head-url"] = target_to_url(head)
         opts["head-path"] = url_to_rlpath(opts["head-url"])