diff --git a/src/libstore/outputs-spec.cc b/src/libstore/outputs-spec.cc
index a9e4320d5..096443cb2 100644
--- a/src/libstore/outputs-spec.cc
+++ b/src/libstore/outputs-spec.cc
@@ -21,7 +21,8 @@ bool OutputsSpec::contains(const std::string & outputName) const
 
 std::optional<OutputsSpec> OutputsSpec::parseOpt(std::string_view s)
 {
-    static std::regex regex(R"((\*)|([a-z]+(,[a-z]+)*))");
+    // See checkName() for valid output name characters.
+    static std::regex regex(R"((\*)|([a-zA-Z\+\-\._\?=]+(,[a-zA-Z\+\-\._\?=]+)*))");
 
     std::smatch match;
     std::string s2 { s }; // until some improves std::regex
@@ -42,7 +43,7 @@ OutputsSpec OutputsSpec::parse(std::string_view s)
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid outputs specifier: '%s'", s);
+        throw Error("invalid outputs specifier '%s'", s);
     return *spec;
 }
 
@@ -65,7 +66,7 @@ std::pair<std::string_view, ExtendedOutputsSpec> ExtendedOutputsSpec::parse(std:
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid extended outputs specifier: '%s'", s);
+        throw Error("invalid extended outputs specifier '%s'", s);
     return *spec;
 }
 
diff --git a/tests/build.sh b/tests/build.sh
index 036fb037e..898c6963a 100644
--- a/tests/build.sh
+++ b/tests/build.sh
@@ -42,20 +42,21 @@ nix build -f multiple-outputs.nix --json 'a^*' --no-link | jq --exit-status '
 nix build -f multiple-outputs.nix --json e --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b"]))
+    (.outputs | keys == ["a_a", "b"]))
 '
 
 # But not when it's overriden.
-nix build -f multiple-outputs.nix --json e^a --no-link | jq --exit-status '
+nix build -f multiple-outputs.nix --json e^a_a --no-link
+nix build -f multiple-outputs.nix --json e^a_a --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a"]))
+    (.outputs | keys == ["a_a"]))
 '
 
 nix build -f multiple-outputs.nix --json 'e^*' --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b", "c"]))
+    (.outputs | keys == ["a_a", "b", "c"]))
 '
 
 # Test building from raw store path to drv not expression.
@@ -104,7 +105,7 @@ nix build "$drv^*" --no-link --json | jq --exit-status '
 nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b"]))
+    (.outputs | keys == ["a_a", "b"]))
 '
 
 testNormalization () {
diff --git a/tests/multiple-outputs.nix b/tests/multiple-outputs.nix
index 1429bc648..9a097b5f1 100644
--- a/tests/multiple-outputs.nix
+++ b/tests/multiple-outputs.nix
@@ -91,9 +91,9 @@ rec {
 
   e = mkDerivation {
     name = "multiple-outputs-e";
-    outputs = [ "a" "b" "c" ];
-    meta.outputsToInstall = [ "a" "b" ];
-    buildCommand = "mkdir $a $b $c";
+    outputs = [ "a_a" "b" "c" ];
+    meta.outputsToInstall = [ "a_a" "b" ];
+    buildCommand = "mkdir $a_a $b $c";
   };
 
   independent = mkDerivation {