diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index 9d07a7a85..0ee533e85 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -313,7 +313,7 @@ struct Common : InstallableCommand, MixProfile
         buildEnvironment.toBash(out, ignoreVars);
 
         for (auto & var : savedVars)
-            out << fmt("%s=\"$%s:$nix_saved_%s\"\n", var, var, var);
+            out << fmt("%s=\"$%s${nix_saved_%s:+:$nix_saved_%s}\"\n", var, var, var, var);
 
         out << "export NIX_BUILD_TOP=\"$(mktemp -d -t nix-shell.XXXXXX)\"\n";
         for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
diff --git a/tests/nix-shell.sh b/tests/nix-shell.sh
index a6e32ed54..044b96d54 100644
--- a/tests/nix-shell.sh
+++ b/tests/nix-shell.sh
@@ -94,16 +94,37 @@ echo foo | nix develop -f "$shellDotNix" shellDrv -c cat | grepQuiet foo
 nix develop -f "$shellDotNix" shellDrv -c echo foo |& grepQuiet foo
 
 # Test 'nix print-dev-env'.
-[[ $(nix print-dev-env -f "$shellDotNix" shellDrv --json | jq -r .variables.arr1.value[2]) = '3 4' ]]
+
+nix print-dev-env -f "$shellDotNix" shellDrv > $TEST_ROOT/dev-env.sh
+nix print-dev-env -f "$shellDotNix" shellDrv --json > $TEST_ROOT/dev-env.json
+
+# Ensure `nix print-dev-env --json` contains variable assignments.
+[[ $(jq -r .variables.arr1.value[2] $TEST_ROOT/dev-env.json) = '3 4' ]]
+
+# Run tests involving `source <(nix print-dev-inv)` in subshells to avoid modifying the current
+# environment.
 
 set +u # FIXME: Make print-dev-env `set -u` compliant (issue #7951)
 
-source <(nix print-dev-env -f "$shellDotNix" shellDrv)
-[[ -n $stdenv ]]
-[[ ${arr1[2]} = "3 4" ]]
-[[ ${arr2[1]} = $'\n' ]]
-[[ ${arr2[2]} = $'x\ny' ]]
-[[ $(fun) = blabla ]]
+# Ensure `source <(nix print-dev-env)` modifies the environment.
+(
+    path=$PATH
+    source $TEST_ROOT/dev-env.sh
+    [[ -n $stdenv ]]
+    [[ ${arr1[2]} = "3 4" ]]
+    [[ ${arr2[1]} = $'\n' ]]
+    [[ ${arr2[2]} = $'x\ny' ]]
+    [[ $(fun) = blabla ]]
+    [[ $PATH = $(jq -r .variables.PATH.value $TEST_ROOT/dev-env.json):$path ]]
+)
+
+# Ensure `source <(nix print-dev-env)` handles the case when PATH is empty.
+(
+    path=$PATH
+    PATH=
+    source $TEST_ROOT/dev-env.sh
+    [[ $PATH = $(PATH=$path jq -r .variables.PATH.value $TEST_ROOT/dev-env.json) ]]
+)
 
 # Test nix-shell with ellipsis and no `inNixShell` argument (for backwards compat with old nixpkgs)
 cat >$TEST_ROOT/shell-ellipsis.nix <<EOF