diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index a9b6a8dbf..612434e4d 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2090,10 +2090,11 @@ void LocalDerivationGoal::runChild()
 
             /* The tmpDir in scope points at the temporary build directory for our derivation. Some packages try different mechanisms
                to find temporary directories, so we want to open up a broader place for them to dump their files, if needed. */
-            Path globalTmpDir = canonPath(getEnvNonEmpty("TMPDIR").value_or("/tmp"), true);
+            Path globalTmpDir = canonPath(defaultTempDir(), true);
 
             /* They don't like trailing slashes on subpath directives */
-            if (globalTmpDir.back() == '/') globalTmpDir.pop_back();
+            while (!globalTmpDir.empty() && globalTmpDir.back() == '/')
+                globalTmpDir.pop_back();
 
             if (getEnv("_NIX_TEST_NO_SANDBOX") != "1") {
                 builder = "/usr/bin/sandbox-exec";
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index d22ae4ca0..fa0938d7b 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -415,7 +415,7 @@ void initLibStore() {
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
-    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
+    if (hasPrefix(defaultTempDir(), "/var/folders/"))
         unsetenv("TMPDIR");
 #endif
 
diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index 85c5eed4c..5da87e935 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -49,7 +49,7 @@ public:
         , BinaryCacheStore(params)
         , cacheUri(scheme + "://" + _cacheUri)
     {
-        if (cacheUri.back() == '/')
+        while (!cacheUri.empty() && cacheUri.back() == '/')
             cacheUri.pop_back();
 
         diskCache = getNarInfoDiskCache();
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index 9dd6a5133..9f81ee452 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -494,10 +494,14 @@ void AutoDelete::reset(const Path & p, bool recursive) {
 
 //////////////////////////////////////////////////////////////////////
 
+std::string defaultTempDir() {
+    return getEnvNonEmpty("TMPDIR").value_or("/tmp");
+}
+
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
     std::atomic<unsigned int> & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
+    tmpRoot = canonPath(tmpRoot.empty() ? defaultTempDir() : tmpRoot, true);
     if (includePid)
         return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
     else
@@ -537,7 +541,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
-    Path tmpl(getEnv("TMPDIR").value_or("/tmp") + "/" + prefix + ".XXXXXX");
+    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
     // FIXME: use O_TMPFILE.
     AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index 963265e34..9d565c881 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -234,6 +234,10 @@ Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
  */
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
 
+/**
+ * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
 
 /**
  * Used in various places.
diff --git a/src/libutil/git.cc b/src/libutil/git.cc
index 0b6e35222..a60589baa 100644
--- a/src/libutil/git.cc
+++ b/src/libutil/git.cc
@@ -251,6 +251,7 @@ void dumpTree(const Tree & entries, Sink & sink,
     for (auto & [name, entry] : entries) {
         auto name2 = name;
         if (entry.mode == Mode::Directory) {
+            assert(!name2.empty());
             assert(name2.back() == '/');
             name2.pop_back();
         }
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index a372e4b1c..35eef5b83 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -475,8 +475,9 @@ static void main_nix_build(int argc, char * * argv)
         // Set the environment.
         auto env = getEnv();
 
-        auto tmp = getEnv("TMPDIR");
-        if (!tmp) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
+        auto tmp = getEnvNonEmpty("TMPDIR");
+        if (!tmp)
+            tmp = getEnvNonEmpty("XDG_RUNTIME_DIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;