diff --git a/doc/build-unix.md b/doc/build-unix.md
--- a/doc/build-unix.md
+++ b/doc/build-unix.md
@@ -54,7 +54,7 @@
 memory available when compiling Bitcoin ABC. On systems with less, gcc can be
 tuned to conserve memory with additional CXXFLAGS:
 
-    cmake -GNinja .. -DCXXFLAGS="--param ggc-min-expand=1 --param ggc-min-heapsize=32768"
+    cmake -GNinja .. -DCMAKE_CXX_FLAGS="--param ggc-min-expand=1 --param ggc-min-heapsize=32768"
 
 Dependency Build Instructions: Ubuntu & Debian
 ----------------------------------------------