Make sure our default R version is valid semver

repo2docker/buildpacks/r.py

@@ -74,7 +74,8 @@ class RBuildPack(PythonBuildPack):
         }
 
         # the default if nothing is specified
-        r_version = "4.1"
+        # Use full version is needed here, so it a valid semver
+        r_version = version_map["4.1"]
 
         if not hasattr(self, "_r_version"):
             parts = self.runtime.split("-")