diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 22c3303ad9..83d0864356 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -156,6 +156,10 @@ jobs:
         export LIBRARY_PATH="$LIBRARY_PATH:/usr/local/opt/openssl/lib/"
     - name: Build V
       run:  make -j4 && ./v -cg -cflags -Werror -o v cmd/v
+    - name: Run sanitizers
+      run: |
+        ./v -o v2 cmd/v -cflags -fsanitize=undefined
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v2 -o v.c cmd/v
     - name: Build V using V
       run:  ./v -o v2 cmd/v && ./v2 -o v3 cmd/v
     - name: v self with -usecache
@@ -221,6 +225,12 @@ jobs:
       run: make -j4 && ./v -cc gcc -cg -cflags -Werror -o v cmd/v
     - name: Valgrind
       run: valgrind --error-exitcode=1 ./v -o v.c cmd/v
+    - name: Run sanitizers
+      run: |
+        ./v -o v2 cmd/v -cflags -fsanitize=thread
+        ./v -o v3 cmd/v -cflags "-fsanitize=undefined -fno-sanitize=alignment"
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v2 -o v.c cmd/v
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v3 -o v.c cmd/v
 #    - name: Test V
 #      run: ./v -silent test-compiler
 #    - name: Test v binaries
@@ -310,8 +320,12 @@ jobs:
       run: valgrind --error-exitcode=1 ./v -o v.c cmd/v
     - name: Run sanitizers
       run: |
-        ./v -o v2 cmd/v -cflags -fsanitize=memory && ./v2 -o v.c cmd/v
-        ./v -o v2 cmd/v -cflags -fsanitize=thread && ./v2 -o v.c cmd/v
+        ./v -o v2 cmd/v -cflags -fsanitize=memory
+        ./v -o v3 cmd/v -cflags -fsanitize=thread
+        ./v -o v4 cmd/v -cflags -fsanitize=undefined
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v2 -o v.c cmd/v
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v3 -o v.c cmd/v
+        UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ./v4 -o v.c cmd/v
     - name: v self compilation
       run: ./v -o v2 cmd/v && ./v2 -o v3 cmd/v && ./v3 -o v4 cmd/v
     - name: v self with -usecache
diff --git a/vlib/builtin/array.v b/vlib/builtin/array.v
index f2f4842254..e15b4f642d 100644
--- a/vlib/builtin/array.v
+++ b/vlib/builtin/array.v
@@ -338,7 +338,9 @@ pub fn (a &array) clone() array {
 			unsafe {arr.set_unsafe(i, &ar_clone)}
 		}
 	} else {
-		unsafe {C.memcpy(byteptr(arr.data), a.data, a.cap * a.element_size)}
+		if !isnil(a.data) {
+			unsafe {C.memcpy(byteptr(arr.data), a.data, a.cap * a.element_size)}
+		}
 	}
 	return arr
 }
@@ -396,7 +398,7 @@ fn (mut a array) push(val voidptr) {
 // `val` is array.data
 // TODO make private, right now it's used by strings.Builder
 pub fn (mut a3 array) push_many(val voidptr, size int) {
-	if a3.data == val {
+	if a3.data == val && !isnil(a3.data) {
 		// handle `arr << arr`
 		copy := a3.clone()
 		a3.ensure_cap(a3.len + size)
@@ -406,7 +408,9 @@ pub fn (mut a3 array) push_many(val voidptr, size int) {
 		}
 	} else {
 		a3.ensure_cap(a3.len + size)
-		unsafe {C.memcpy(a3.get_unsafe(a3.len), val, a3.element_size * size)}
+		if !isnil(a3.data) && !isnil(val) {
+			unsafe {C.memcpy(a3.get_unsafe(a3.len), val, a3.element_size * size)}
+		}
 	}
 	a3.len += size
 }
diff --git a/vlib/strconv/atof.v b/vlib/strconv/atof.v
index 325893f411..4a60d9af22 100644
--- a/vlib/strconv/atof.v
+++ b/vlib/strconv/atof.v
@@ -356,7 +356,7 @@ fn converter(mut pn PrepNumber) u64 {
 	mut r2 := u32(0) // 96-bit precision integer
 	mut r1 := u32(0)
 	mut r0 := u32(0)
-	mask28 := u32(0xF<<28)
+	mask28 := u32(u64(0xF)<<28)
 	mut result := u64(0)
 	// working on 3 u32 to have 96 bit precision
 	s0 = u32(pn.mantissa & u64(0x00000000FFFFFFFF))