diff --git a/src/CodeGenerator.cpp b/src/CodeGenerator.cpp
index c1e2dc6..3fd43e1 100644
--- a/src/CodeGenerator.cpp
+++ b/src/CodeGenerator.cpp
@@ -3,6 +3,7 @@
 #include "settings.hpp"
 
 #include <algorithm>
+#include <limits>
 
 using namespace std;
 
@@ -161,7 +162,7 @@ static const char* ctx_of_dw_name(SimpleDwarf::MachineRegister reg) {
 void CodeGenerator::gen_of_reg(const SimpleDwarf::DwRegister& reg) {
     switch(reg.type) {
         case SimpleDwarf::DwRegister::REG_UNDEFINED:
-            os << "0";  // FIXME do better?
+            os << std::numeric_limits<uintptr_t>::max() << "ull";
             break;
         case SimpleDwarf::DwRegister::REG_REGISTER:
             os << ctx_of_dw_name(reg.reg)
diff --git a/stack_walker/stack_walker.cpp b/stack_walker/stack_walker.cpp
index 0d8f3ef..f8feece 100644
--- a/stack_walker/stack_walker.cpp
+++ b/stack_walker/stack_walker.cpp
@@ -224,6 +224,9 @@ bool unwind_context(unwind_context_t& ctx) {
 
     uintptr_t tr_pc = ctx.rip - mmap_entry->beg;
     ctx = fde_func(ctx, tr_pc);
+
+    if(ctx.rbp == 0 || ctx.rip + 1 == 0)
+        return false;
     return true;
 }