diff --git a/src/libutil/canon-path.cc b/src/libutil/canon-path.cc
index 7aeabe8d9..79951c933 100644
--- a/src/libutil/canon-path.cc
+++ b/src/libutil/canon-path.cc
@@ -16,14 +16,13 @@ CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
 std::optional<CanonPath> CanonPath::parent() const
 {
     if (isRoot()) return std::nullopt;
-    return CanonPath(unchecked_t(), path.substr(0, path.rfind('/')));
+    return CanonPath(unchecked_t(), path.substr(0, std::max((size_t) 1, path.rfind('/'))));
 }
 
 void CanonPath::pop()
 {
     assert(!isRoot());
-    auto slash = path.rfind('/');
-    path.resize(std::max((size_t) 1, slash));
+    path.resize(std::max((size_t) 1, path.rfind('/')));
 }
 
 bool CanonPath::isWithin(const CanonPath & parent) const
diff --git a/src/libutil/tests/canon-path.cc b/src/libutil/tests/canon-path.cc
index 28fa789a0..c1c5adadf 100644
--- a/src/libutil/tests/canon-path.cc
+++ b/src/libutil/tests/canon-path.cc
@@ -11,6 +11,7 @@ namespace nix {
             ASSERT_EQ(p.rel(), "");
             ASSERT_EQ(p.baseName(), std::nullopt);
             ASSERT_EQ(p.dirOf(), std::nullopt);
+            ASSERT_FALSE(p.parent());
         }
 
         {
@@ -19,6 +20,7 @@ namespace nix {
             ASSERT_EQ(p.rel(), "foo");
             ASSERT_EQ(*p.baseName(), "foo");
             ASSERT_EQ(*p.dirOf(), ""); // FIXME: do we want this?
+            ASSERT_EQ(p.parent()->abs(), "/");
         }
 
         {
@@ -27,6 +29,7 @@ namespace nix {
             ASSERT_EQ(p.rel(), "foo/bar");
             ASSERT_EQ(*p.baseName(), "bar");
             ASSERT_EQ(*p.dirOf(), "/foo");
+            ASSERT_EQ(p.parent()->abs(), "/foo");
         }
 
         {