diff --git a/debug/tst-backtrace2.c b/debug/tst-backtrace2.c
index 846ca35..396d743 100644
--- a/debug/tst-backtrace2.c
+++ b/debug/tst-backtrace2.c
@@ -94,6 +94,12 @@ fn3 (void)
 NO_INLINE static int
 do_test (void)
 {
+  /* Test BZ #18084.  */
+  void *buffer[1];
+
+  if (backtrace (buffer, 0) != 0)
+    FAIL ();
+
   fn3 ();
   return ret;
 }
diff --git a/sysdeps/arm/backtrace.c b/sysdeps/arm/backtrace.c
index 7ab487c..5a30c20 100644
--- a/sysdeps/arm/backtrace.c
+++ b/sysdeps/arm/backtrace.c
@@ -92,6 +92,10 @@ __backtrace (array, size)
      int size;
 {
   struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -100,8 +104,7 @@ __backtrace (array, size)
     return 0;
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   if (arg.cnt > 1 && arg.array[arg.cnt - 1] == NULL)
     --arg.cnt;
diff --git a/sysdeps/i386/backtrace.c b/sysdeps/i386/backtrace.c
index 550234f..f10ed56 100644
--- a/sysdeps/i386/backtrace.c
+++ b/sysdeps/i386/backtrace.c
@@ -114,6 +114,10 @@ __backtrace (array, size)
      int size;
 {
   struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -122,8 +126,7 @@ __backtrace (array, size)
     return 0;
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   if (arg.cnt > 1 && arg.array[arg.cnt - 1] == NULL)
     --arg.cnt;
diff --git a/sysdeps/m68k/backtrace.c b/sysdeps/m68k/backtrace.c
index 3114251..ca7d259 100644
--- a/sysdeps/m68k/backtrace.c
+++ b/sysdeps/m68k/backtrace.c
@@ -111,6 +111,10 @@ int
 __backtrace (void **array, int size)
 {
   struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -119,8 +123,7 @@ __backtrace (void **array, int size)
     return 0;
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   if (arg.cnt > 1 && arg.array[arg.cnt - 1] == NULL)
     --arg.cnt;
diff --git a/sysdeps/microblaze/backtrace.c b/sysdeps/microblaze/backtrace.c
index 24e827a..c043ba7 100644
--- a/sysdeps/microblaze/backtrace.c
+++ b/sysdeps/microblaze/backtrace.c
@@ -114,6 +114,9 @@ __backtrace (void **array, int size)
   int count;
   int rc = 0;
 
+  if (size <= 0)
+    return 0;
+
   __asm__ __volatile__ ("mfs %0, rpc"
                         : "=r"(pc));
 
diff --git a/sysdeps/s390/s390-32/backtrace.c b/sysdeps/s390/s390-32/backtrace.c
index 73db652..4e5e1fe 100644
--- a/sysdeps/s390/s390-32/backtrace.c
+++ b/sysdeps/s390/s390-32/backtrace.c
@@ -126,6 +126,10 @@ int
 __backtrace (void **array, int size)
 {
   struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -135,8 +139,7 @@ __backtrace (void **array, int size)
     return __backchain_backtrace (array, size);
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   return arg.cnt != -1 ? arg.cnt : 0;
 }
diff --git a/sysdeps/s390/s390-64/backtrace.c b/sysdeps/s390/s390-64/backtrace.c
index 08e563e..184a737 100644
--- a/sysdeps/s390/s390-64/backtrace.c
+++ b/sysdeps/s390/s390-64/backtrace.c
@@ -125,6 +125,10 @@ int
 __backtrace (void **array, int size)
 {
   struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -134,8 +138,7 @@ __backtrace (void **array, int size)
     return __backchain_backtrace (array, size);
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   return arg.cnt != -1 ? arg.cnt : 0;
 }
diff --git a/sysdeps/sparc/backtrace.c b/sysdeps/sparc/backtrace.c
index 754f45b..cb01fd6 100644
--- a/sysdeps/sparc/backtrace.c
+++ b/sysdeps/sparc/backtrace.c
@@ -108,7 +108,7 @@ __backtrace (void **array, int size)
   bool use_unwinder;
   int count;
 
-  if (!size)
+  if (size <= 0)
     return 0;
 
   use_unwinder = true;
diff --git a/sysdeps/x86_64/backtrace.c b/sysdeps/x86_64/backtrace.c
index 2a3848d..21448c8 100644
--- a/sysdeps/x86_64/backtrace.c
+++ b/sysdeps/x86_64/backtrace.c
@@ -97,6 +97,10 @@ __backtrace (array, size)
      int size;
 {
   struct trace_arg arg = { .array = array, .cfa = 0, .size = size, .cnt = -1 };
+
+  if (size <= 0)
+    return 0;
+
 #ifdef SHARED
   __libc_once_define (static, once);
 
@@ -105,8 +109,7 @@ __backtrace (array, size)
     return 0;
 #endif
 
-  if (size >= 1)
-    unwind_backtrace (backtrace_helper, &arg);
+  unwind_backtrace (backtrace_helper, &arg);
 
   /* _Unwind_Backtrace seems to put NULL address above
      _start.  Fix it up here.  */