diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index f7f3204a4..e45f5af81 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -16,6 +16,7 @@
 #include "cgroup.hh"
 #include "personality.hh"
 #include "namespaces.hh"
+#include "file-system.hh"
 
 #include <regex>
 #include <queue>
@@ -2116,10 +2117,11 @@ 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 9c25d9868..f6a3fefca 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "abstract-setting-to-json.hh"
 #include "compute-levels.hh"
+#include "file-system.hh"
 
 #include <algorithm>
 #include <map>
@@ -408,7 +409,7 @@ 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.hh b/src/libutil/file-system.hh
new file mode 100644
index 000000000..1d404028b
--- /dev/null
+++ b/src/libutil/file-system.hh
@@ -0,0 +1,17 @@
+#pragma once
+/**
+ * @file
+ *
+ * Utiltities for working with the file sytem and file paths.
+ */
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
+
+}
diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc
index 2a7787c0e..17c3cb8bf 100644
--- a/src/libutil/filesystem.cc
+++ b/src/libutil/filesystem.cc
@@ -5,15 +5,20 @@
 #include "finally.hh"
 #include "util.hh"
 #include "types.hh"
+#include "file-system.hh"
 
 namespace fs = std::filesystem;
 
 namespace nix {
 
+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
@@ -53,7 +58,7 @@ 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/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index e2189fc66..6c378b244 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -457,8 +457,9 @@ 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;