diff --git a/docs/writingmodules.rst b/docs/writingmodules.rst
index baeaa0c7c8..0e20f0c359 100644
--- a/docs/writingmodules.rst
+++ b/docs/writingmodules.rst
@@ -657,11 +657,14 @@ checks in your code nevertheless). In those cases you can use the
         const uint8_t* block_data;
 
         block = first_memory_block(context);
-        block_data = block->fetch_data(block)
-
-        if (block_data != NULL)
+        if (block != NULL)
         {
-          ..do something with the memory block
+          block_data = block->fetch_data(block)
+
+          if (block_data != NULL)
+          {
+            ..do something with the memory block
+          }
         }
     }
 
diff --git a/libyara/modules/magic/magic.c b/libyara/modules/magic/magic.c
index 5e435d8f41..03e9f31d8a 100644
--- a/libyara/modules/magic/magic.c
+++ b/libyara/modules/magic/magic.c
@@ -101,6 +101,10 @@ define_function(magic_mime_type)
   if (cache->cached_mime_type == NULL)
   {
     block = first_memory_block(context);
+
+    if (block == NULL)
+      return_string(YR_UNDEFINED);
+
     block_data = block->fetch_data(block);
 
     if (block_data != NULL)
@@ -134,6 +138,10 @@ define_function(magic_type)
   if (cache->cached_type == NULL)
   {
     block = first_memory_block(context);
+
+    if (block == NULL)
+      return_string(YR_UNDEFINED);
+
     block_data = block->fetch_data(block);
 
     if (block_data != NULL)
diff --git a/libyara/modules/math/math.c b/libyara/modules/math/math.c
index 98193c4a63..ee1e072d2f 100644
--- a/libyara/modules/math/math.c
+++ b/libyara/modules/math/math.c
@@ -64,7 +64,7 @@ uint32_t* get_distribution(int64_t offset, int64_t length, YR_SCAN_CONTEXT* cont
   YR_MEMORY_BLOCK* block = first_memory_block(context);
   YR_MEMORY_BLOCK_ITERATOR* iterator = context->iterator;
 
-  if (offset < 0 || length < 0 || offset < block->base)
+  if (block == NULL || offset < 0 || length < 0 || offset < block->base)
   {
     yr_free(data);
     return NULL;
@@ -132,9 +132,8 @@ uint32_t* get_distribution_global(YR_SCAN_CONTEXT* context) {
   if (data == NULL)
     return NULL;
 
-  YR_MEMORY_BLOCK* block = first_memory_block(context);
+  YR_MEMORY_BLOCK* block;
   YR_MEMORY_BLOCK_ITERATOR* iterator = context->iterator;
-
   foreach_memory_block(iterator, block)
   {
     if (expected_next_offset != block->base)
@@ -323,6 +322,9 @@ define_function(data_serial_correlation)
   YR_MEMORY_BLOCK* block = first_memory_block(context);
   YR_MEMORY_BLOCK_ITERATOR* iterator = context->iterator;
 
+  if (block == NULL)
+    return_float(YR_UNDEFINED);
+
   double sccun = 0;
   double sccfirst = 0;
   double scclast = 0;
@@ -450,6 +452,9 @@ define_function(data_monte_carlo_pi)
   YR_MEMORY_BLOCK* block = first_memory_block(context);
   YR_MEMORY_BLOCK_ITERATOR* iterator = context->iterator;
 
+  if (block == NULL)
+    return_float(YR_UNDEFINED);
+
   if (offset < 0 || length < 0 || offset < block->base)
     return_float(YR_UNDEFINED);