diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index e4210c0cf..05fd301e1 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -434,7 +434,7 @@ LockedFlake lockFlake(
                         if (auto relativePath = input.ref->input.isRelative()) {
                             SourcePath inputSourcePath {
                                 overridenSourcePath.accessor,
-                                *overridenSourcePath.path.parent() + *relativePath
+                                CanonPath(*relativePath, *overridenSourcePath.path.parent())
                             };
                             return readFlake(state, *input.ref, *input.ref, *input.ref, inputSourcePath, inputPath);
                         } else
diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 2c958276b..98dcf38e4 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -95,7 +95,7 @@ bool Input::isLocked() const
     return scheme && scheme->isLocked(*this);
 }
 
-std::optional<CanonPath> Input::isRelative() const
+std::optional<std::string> Input::isRelative() const
 {
     assert(scheme);
     return scheme->isRelative(*this);
diff --git a/src/libfetchers/fetchers.hh b/src/libfetchers/fetchers.hh
index a890a5ef0..c2753da30 100644
--- a/src/libfetchers/fetchers.hh
+++ b/src/libfetchers/fetchers.hh
@@ -53,7 +53,7 @@ public:
 
     /* Only for relative path flakes, i.e. 'path:./foo', returns the
        relative path, i.e. './foo'. */
-    std::optional<CanonPath> isRelative() const;
+    std::optional<std::string> isRelative() const;
 
     bool operator ==(const Input & other) const;
 
@@ -138,7 +138,7 @@ struct InputScheme
     virtual bool isLocked(const Input & input) const
     { return false; }
 
-    virtual std::optional<CanonPath> isRelative(const Input & input) const
+    virtual std::optional<std::string> isRelative(const Input & input) const
     { return std::nullopt; }
 
     virtual std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const;
diff --git a/src/libfetchers/path.cc b/src/libfetchers/path.cc
index 72596841f..96e34af79 100644
--- a/src/libfetchers/path.cc
+++ b/src/libfetchers/path.cc
@@ -78,13 +78,13 @@ struct PathInputScheme : InputScheme
         };
     }
 
-    std::optional<CanonPath> isRelative(const Input & input) const override
+    std::optional<std::string> isRelative(const Input & input) const override
     {
         auto path = getStrAttr(input.attrs, "path");
         if (hasPrefix(path, "/"))
             return std::nullopt;
         else
-            return CanonPath(path);
+            return path;
     }
 
     bool isLocked(const Input & input) const override
diff --git a/src/libutil/canon-path.cc b/src/libutil/canon-path.cc
index 79951c933..b132b4262 100644
--- a/src/libutil/canon-path.cc
+++ b/src/libutil/canon-path.cc
@@ -61,6 +61,7 @@ CanonPath CanonPath::operator + (const CanonPath & x) const
 void CanonPath::push(std::string_view c)
 {
     assert(c.find('/') == c.npos);
+    assert(c != "." && c != "..");
     if (!isRoot()) path += '/';
     path += c;
 }