diff --git a/var/spack/repos/builtin/packages/r-sf/package.py b/var/spack/repos/builtin/packages/r-sf/package.py
index 68e61eb1c70f35e196b67a2fa629155cee02de0c..711915fc6f79aa5f8244ce7d851d78dd4f7ad264 100644
--- a/var/spack/repos/builtin/packages/r-sf/package.py
+++ b/var/spack/repos/builtin/packages/r-sf/package.py
@@ -25,4 +25,6 @@ class RSf(RPackage):
     depends_on('r-magrittr', type=('build', 'run'))
     depends_on('gdal@2.0.0:')
     depends_on('geos@3.3.0:')
-    depends_on('proj@4.8.0:')
+    # Since PROJ.4 and PROJ.6 have incompatible APIs, I'm assuming
+    # that since r-sf supports PROJ.4, it doesn't support PROJ.6
+    depends_on('proj@4.8.0:5')