diff --git a/giteapc/repo.py b/giteapc/repo.py
index dcea81d..44ca935 100644
--- a/giteapc/repo.py
+++ b/giteapc/repo.py
@@ -105,7 +105,7 @@ def pretty_repo(r):
 
 class Spec:
     # A spec in REPOSITORY[@VERSION][:CONFIGURATION]
-    RE_SPEC = re.compile(r'^([^@:]+)(?:@([^@:]+))?(?:[:]([^@:]+))?')
+    RE_SPEC = re.compile(r'^([^@:]+)(?:@([^@:]+))?(?:([:])([^@:]*))?')
 
     def __init__(self, string):
         m = re.match(Spec.RE_SPEC, string)
@@ -114,7 +114,7 @@ class Spec:
 
         self.name    = m[1]
         self.version = m[2]
-        self.config  = m[3]
+        self.config  = m[4] or (m[3] and "")
         self.repo    = None
 
     def resolve(self, local_only=False, remote_only=False):
@@ -286,7 +286,7 @@ def build(*args, install=False, skip_configure=False):
             raise Error(f"{r.fullname} has no giteapc.make")
 
         env = os.environ.copy()
-        if s.config:
+        if s.config is not None:
             env["GITEAPC_CONFIG"] = s.config
             r.set_config(s.config)
         env["GITEAPC_PREFIX"] = PREFIX_FOLDER
diff --git a/giteapc/repos.py b/giteapc/repos.py
index d223d22..cafe0d3 100644
--- a/giteapc/repos.py
+++ b/giteapc/repos.py
@@ -165,8 +165,19 @@ class LocalRepo:
         source = self.folder + f"/giteapc-config.make"
         target = self.folder + f"/giteapc-config-{config}.make"
 
-        if os.path.exists(source):
+        if config == "":
+            if os.path.islink(source):
+                os.remove(source)
+            return
+
+        if not os.path.exists(target):
+            raise Error(f"config '{config}' does not exist")
+
+        if os.path.islink(source):
             os.remove(source)
+        elif os.path.exists(source):
+            raise Error("giteapc-config.make is not a link, was it altered?")
+
         os.symlink(target, source)
 
     # Metadata