diff --git a/crossbuild-runner/backends/x86-64/posix-headers.lisp b/crossbuild-runner/backends/x86-64/posix-headers.lisp
index 1d2b84c4d..0968a0bdd 100644
--- a/crossbuild-runner/backends/x86-64/posix-headers.lisp
+++ b/crossbuild-runner/backends/x86-64/posix-headers.lisp
@@ -163,6 +163,7 @@
 
 (defconstant sizeof-timespec 16) ; #x10
 (defconstant sizeof-timeval 16) ; #x10
+(defconstant sizeof-unw-cursor 0) ; #x0
 
 ;;; sysctl(3) names
 (in-package "SB-IMPL")
diff --git a/crossbuild-runner/backends/x86-64/win32-headers.lisp b/crossbuild-runner/backends/x86-64/win32-headers.lisp
index cfba763a4..fc889d6e1 100644
--- a/crossbuild-runner/backends/x86-64/win32-headers.lisp
+++ b/crossbuild-runner/backends/x86-64/win32-headers.lisp
@@ -206,6 +206,7 @@
 
 (defconstant sizeof-timespec 16) ; #x10
 (defconstant sizeof-timeval 8) ; #x8
+(defconstant sizeof-unw-cursor 0) ; #x0
 
 (in-package "SB-KERNEL")