diff --git a/pkgs/development/libraries/qt-5/5.4/qtbase.nix b/pkgs/development/libraries/qt-5/5.4/qtbase.nix
index cf960c39b40..441dbdd5c66 100644
--- a/pkgs/development/libraries/qt-5/5.4/qtbase.nix
+++ b/pkgs/development/libraries/qt-5/5.4/qtbase.nix
@@ -143,6 +143,11 @@ stdenv.mkDerivation {
     -${optionalString (buildTests == false) "no"}make tests
   '';
 
+  # PostgreSQL autodetection fails sporadically because Qt omits the "-lpq" flag
+  # if dependency paths contain the string "pq", which can occur in the hash.
+  # To prevent these failures, we need to override PostgreSQL detection.
+  PSQL_LIBS = optionalString (postgresql != null) "-L${postgresql}/lib -lpq";
+
   propagatedBuildInputs = [
     xlibs.libXcomposite libX11 libxcb libXext libXrender libXi
     fontconfig freetype openssl dbus.libs glib udev libxml2 libxslt pcre