diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 715608ad2..903eacf5e 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1998,6 +1998,11 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         } else if (firstType == nPath) {
             if (!first) {
                 auto part = state.coerceToString(i_pos, *vTmp, context, false, false);
+                if (sSize <= 1 && !hasPrefix(*part, "/"))
+                    state.throwEvalError(i_pos,
+                        "cannot append non-absolute path '%1%' to '%2%' (hint: change it to '/%1%')",
+                        (std::string) *part, accessor->root().to_string(),
+                        env, *this);
                 sSize += part->size();
                 s.emplace_back(std::move(part));
             }
diff --git a/tests/toString-path.sh b/tests/toString-path.sh
index c17dc262e..1c469ace6 100644
--- a/tests/toString-path.sh
+++ b/tests/toString-path.sh
@@ -3,6 +3,10 @@ source common.sh
 mkdir -p $TEST_ROOT/foo
 echo bla > $TEST_ROOT/foo/bar
 
-[[ $(nix eval --raw --impure --expr "builtins.readFile (builtins.toString (builtins.fetchTree { type = \"path\"; path = \"$TEST_ROOT/foo\"; } + \"bar\"))") = bla ]]
+[[ $(nix eval --raw --impure --expr "builtins.readFile (builtins.toString (builtins.fetchTree { type = \"path\"; path = \"$TEST_ROOT/foo\"; } + \"/bar\"))") = bla ]]
+
+[[ $(nix eval --raw --impure --expr "builtins.readFile (builtins.toString (builtins.fetchTree { type = \"path\"; path = \"$TEST_ROOT/foo\"; } + \"/b\" + \"ar\"))") = bla ]]
+
+(! nix eval --raw --impure --expr "builtins.fetchTree { type = \"path\"; path = \"$TEST_ROOT/foo\"; } + \"bar\"")
 
 [[ $(nix eval --json --impure --expr "builtins.readDir (builtins.toString (builtins.fetchTree { type = \"path\"; path = \"$TEST_ROOT/foo\"; }))") = '{"bar":"regular"}' ]]