diff --git a/repo2docker/buildpacks/r.py b/repo2docker/buildpacks/r.py
index c3b92739..7d80082a 100644
--- a/repo2docker/buildpacks/r.py
+++ b/repo2docker/buildpacks/r.py
@@ -99,6 +99,9 @@ def r_version(self):
                 # available. Users can however explicitly specify the full version to get something specific
                 if r_version in version_map:
                     r_version = version_map[r_version]
+                elif len(r_version.split(".")) == 1:
+                    # must have x.y.z version, add .0 for unrecognized (future) R versions
+                    r_version += ".0"
 
             # translate to the full version string
             self._r_version = r_version