diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 078a2c41c..582590036 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -243,6 +243,9 @@ static void parse(FileSystemObjectSink & sink, Source & source, const Path & pat
                                         debug("case collision between '%1%' and '%2%'", i->first, name);
                                         name += caseHackSuffix;
                                         name += std::to_string(++i->second);
+                                        auto j = names.find(name);
+                                        if (j != names.end())
+                                            throw Error("NAR contains file name '%s' that collides with case-hacked file name '%s'", prevName, j->first);
                                     } else
                                         names[name] = 0;
                                 }
diff --git a/tests/functional/case-collision.nar b/tests/functional/case-collision.nar
new file mode 100644
index 000000000..2eff86901
Binary files /dev/null and b/tests/functional/case-collision.nar differ
diff --git a/tests/functional/nars.sh b/tests/functional/nars.sh
index 376ef069b..237955fbe 100755
--- a/tests/functional/nars.sh
+++ b/tests/functional/nars.sh
@@ -34,4 +34,10 @@ cmp case.nar "$TEST_ROOT/case.nar"
 # Check whether we detect true collisions (e.g. those remaining after
 # removal of the suffix).
 touch "$TEST_ROOT/case/xt_CONNMARK.h~nix~case~hack~3"
-(! nix-store $opts --dump $TEST_ROOT/case > /dev/null)
+(! nix-store "${opts[@]}" --dump "$TEST_ROOT/case" > /dev/null)
+
+# Detect NARs that have a directory entry that after case-hacking
+# collides with another entry (e.g. a directory containing 'Test',
+# 'Test~nix~case~hack~1' and 'test').
+rm -rf "$TEST_ROOT/case"
+expectStderr 1 nix-store "${opts[@]}" --restore "$TEST_ROOT/case" < case-collision.nar | grepQuiet "NAR contains file name 'test' that collides with case-hacked file name 'Test~nix~case~hack~1'"