diff --git a/src/libexpr/primops/fetchgit.cc b/src/libexpr/primops/fetchgit.cc
index 6a5b5ac36..09e2c077b 100644
--- a/src/libexpr/primops/fetchgit.cc
+++ b/src/libexpr/primops/fetchgit.cc
@@ -58,9 +58,11 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
 
         for (auto & attr : *args[0]->attrs) {
             string name(attr.name);
-            if (name == "url")
-                url = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (name == "rev")
+            if (name == "url") {
+                PathSet context;
+                url = state.coerceToString(*attr.pos, *attr.value, context, false, false);
+                if (hasPrefix(url, "/")) url = "file://" + url;
+            } else if (name == "rev")
                 rev = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
                 throw EvalError("unsupported argument ‘%s’ to ‘fetchgit’, at %s", attr.name, *attr.pos);