diff --git a/common/base_tests.fs b/common/base_tests.fs
index 38a85e0..c8a3f09 100644
--- a/common/base_tests.fs
+++ b/common/base_tests.fs
@@ -122,3 +122,10 @@ e: test-empty-string
   : test s" " ;
   test 0 =assert drop
 ;e
+
+e: test-?dup
+  123 ?dup 123 =assert 123 =assert
+  depth 0 =assert
+  0 ?dup 0 =assert
+  depth 0 =assert
+;e
diff --git a/common/extra_opcodes.h b/common/extra_opcodes.h
index 831fcb6..a0fae87 100644
--- a/common/extra_opcodes.h
+++ b/common/extra_opcodes.h
@@ -25,6 +25,7 @@
   X("-", MINUS, tos = (*sp--) - tos) \
   Y(rot, w = sp[-1]; sp[-1] = *sp; *sp = tos; tos = w) \
   X("-rot", MROT, w = tos; tos = *sp; *sp = sp[-1]; sp[-1] = w) \
+  X("?dup", QDUP, if (tos) DUP) \
   X("<", LESS, tos = (*sp--) < tos ? -1 : 0) \
   X(">", GREATER, tos = (*sp--) > tos ? -1 : 0) \
   X("<=", LESSEQ, tos = (*sp--) <= tos ? -1 : 0) \
diff --git a/common/forth_namespace_tests.fs b/common/forth_namespace_tests.fs
index e88af7e..5f36027 100644
--- a/common/forth_namespace_tests.fs
+++ b/common/forth_namespace_tests.fs
@@ -152,6 +152,7 @@ e: check-extra-opcodes
   out: - 
   out: rot 
   out: -rot 
+  out: ?dup 
   out: < 
   out: > 
   out: <=