diff --git a/Makefile.linux b/Makefile.linux
index c344165f..5b5f87aa 100644
--- a/Makefile.linux
+++ b/Makefile.linux
@@ -34,7 +34,7 @@ ARCH ?= $(shell sh $(SRCDIR)/config-arch.sh)
 
 C_COMPILER_OPTIONS ?= -fno-strict-aliasing -fwrapv -DHAVE_CHICKEN_CONFIG_H
 ifdef DEBUGBUILD
-C_COMPILER_OPTIMIZATION_OPTIONS ?= -g -Wall -Wno-unused -O0 -Wno-cpp
+C_COMPILER_OPTIMIZATION_OPTIONS ?= -g -Wall -Wno-unused -O0 -Wno-cpp -fsanitize=address
 else
 ifdef OPTIMIZE_FOR_SPEED
 C_COMPILER_OPTIMIZATION_OPTIONS ?= -O3 -fomit-frame-pointer
@@ -46,7 +46,7 @@ LINKER_LINK_SHARED_LIBRARY_OPTIONS = -shared
 LINKER_LINK_SHARED_DLOADABLE_OPTIONS = -L. -shared -Wl,-rpath="$(RUNTIME_LINKER_PATH)"
 LINKER_LINK_SHARED_PROGRAM_OPTIONS = -Wl,-rpath="$(RUNTIME_LINKER_PATH)"
 LIBCHICKEN_SO_LINKER_OPTIONS = -Wl,-soname,lib$(PROGRAM_PREFIX)chicken$(PROGRAM_SUFFIX).so.$(BINARYVERSION)
-LIBRARIES = -lm -ldl
+LIBRARIES = -lm -ldl -fsanitize=address
 NEEDS_RELINKING = yes
 USES_SONAME = yes
 
diff --git a/runtime.c b/runtime.c
index bc545def..ea18b43f 100644
--- a/runtime.c
+++ b/runtime.c
@@ -3862,6 +3862,14 @@ static C_regparm void really_mark(C_word *x, C_byte *tgt_space_start, C_byte **t
     if(C_gc_trace_hook != NULL)
       C_gc_trace_hook(x, gc_mode);
 #endif
+    if (debug_mode) {
+      if (val != NULL && C_header_bits(val) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(val), "import0")) {
+        C_dbg(C_text("debug"), C_text("Skipping value1 outside stack/heap/scratch space %p (stack between %p and %p)\n"), val, C_stack_pointer_test, stack_bottom);
+      }
+      if (val != NULL && C_header_bits(val) == C_SYMBOL_TYPE && C_header_bits(C_symbol_name(val)) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(C_symbol_name(val)), "import0")) {
+        C_dbg(C_text("debug"), C_text("Skipping symbol1 outside stack/heap/scratch space %p (stack between %p and %p)\n"), val, C_stack_pointer_test, stack_bottom);
+      }
+    }
     return;
   }
 
@@ -3877,6 +3885,14 @@ static C_regparm void really_mark(C_word *x, C_byte *tgt_space_start, C_byte **t
   /* Already in target space, probably as result of chasing fptrs */
   if ((C_uword)val >= (C_uword)tgt_space_start && (C_uword)val < (C_uword)*tgt_space_top) {
     *x = val;
+    if (debug_mode) {
+      if (C_header_bits(val) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(val), "import0")) {
+        C_dbg(C_text("debug"), C_text("Skipping value2 outside stack/heap/scratch space %p (stack between %p and %p)\n"), val, C_stack_pointer_test, stack_bottom);
+      }
+      if (val != NULL && C_header_bits(val) == C_SYMBOL_TYPE && C_header_bits(C_symbol_name(val)) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(C_symbol_name(val)), "import0")) {
+        C_dbg(C_text("debug"), C_text("Skipping symbol2 outside stack/heap/scratch space %p (stack between %p and %p)\n"), val, C_stack_pointer_test, stack_bottom);
+      }
+    }
     return;
   }
 
@@ -3892,6 +3908,15 @@ static C_regparm void really_mark(C_word *x, C_byte *tgt_space_start, C_byte **t
   n = C_header_size(p);
   bytes = (h & C_BYTEBLOCK_BIT) ? n : n * sizeof(C_word);
 
+  if (debug_mode) {
+    if (C_header_bits(val) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(val), "import0")) {
+      C_dbg(C_text("debug"), C_text("Copying value import0 at %p\n"), val);
+    }
+    if (val != NULL && C_header_bits(val) == C_SYMBOL_TYPE && C_header_bits(C_symbol_name(val)) == C_BYTEVECTOR_TYPE && !strcmp(C_c_string(C_symbol_name(val)), "import0")) {
+      C_dbg(C_text("debug"), C_text("Copying symbol outside stack/heap/scratch space %p (stack between %p and %p)\n"), val, C_stack_pointer_test, stack_bottom);
+    }
+  }
+
   if(C_unlikely(((C_byte *)p2 + bytes + sizeof(C_word)) > tgt_space_limit)) {
     if (gc_mode == GC_MAJOR) {
       /* Detect impossibilities before GC_REALLOC to preserve state: */
diff --git a/utf.c b/utf.c
index d699ccf1..19e4be51 100644
--- a/utf.c
+++ b/utf.c
@@ -3197,18 +3197,30 @@ static C_char *utf8_decode(C_char *buf, C_u32 *c, int *e)
      * shifted out.
      */
     *c  = (C_u32)(s[0] & masks[len]) << 18;
-    *c |= (C_u32)(s[1] & 0x3f) << 12;
-    *c |= (C_u32)(s[2] & 0x3f) <<  6;
-    *c |= (C_u32)(s[3] & 0x3f) <<  0;
+    if (s[0] != '\0') {
+      *c |= (C_u32)(s[1] & 0x3f) << 12;
+      if (s[1] != '\0') {
+        *c |= (C_u32)(s[2] & 0x3f) <<  6;
+        if (s[2] != '\0') {
+          *c |= (C_u32)(s[3] & 0x3f) <<  0;
+        }
+      }
+    }
     *c >>= shiftc[len];
 
     /* Accumulate the various error conditions. */
     *e  = (*c < mins[len]) << 6; // non-canonical encoding
     *e |= ((*c >> 11) == 0x1b) << 7;  // surrogate half?
     *e |= (*c > 0x10FFFF) << 8;  // out of range?
-    *e |= (s[1] & 0xc0) >> 2;
-    *e |= (s[2] & 0xc0) >> 4;
-    *e |= (s[3]       ) >> 6;
+    if (s[0] != '\0') {
+      *e |= (s[1] & 0xc0) >> 2;
+      if (s[1] != '\0') {
+        *e |= (s[2] & 0xc0) >> 4;
+        if (s[2] != '\0') {
+          *e |= (s[3]       ) >> 6;
+        }
+      }
+    }
     *e ^= 0x2a; // top two bits of each tail byte correct?
     *e >>= shifte[len];