diff --git a/libafl/src/executors/forkserver.rs b/libafl/src/executors/forkserver.rs
index fbf85f3f63..0ceecc103f 100644
--- a/libafl/src/executors/forkserver.rs
+++ b/libafl/src/executors/forkserver.rs
@@ -907,7 +907,7 @@ where
                     "Failed to read map size from forkserver".to_string(),
                 ));
             }
-            self.set_map_size(fsrv_map_size);
+            self.set_map_size(fsrv_map_size)?;
         }
 
         if status & FS_NEW_OPT_SHDMEM_FUZZ != 0 {
@@ -973,7 +973,7 @@ where
     ) -> Result<(), Error> {
         if status & FS_OPT_ENABLED == FS_OPT_ENABLED && status & FS_OPT_MAPSIZE == FS_OPT_MAPSIZE {
             let fsrv_map_size = fs_opt_get_mapsize(status);
-            self.set_map_size(fsrv_map_size);
+            self.set_map_size(fsrv_map_size)?;
         }
 
         // Only with SHMEM or AUTODICT we can send send_status back or it breaks!
@@ -1041,7 +1041,7 @@ where
     }
 
     #[allow(clippy::cast_sign_loss)]
-    fn set_map_size(&mut self, fsrv_map_size: i32) {
+    fn set_map_size(&mut self, fsrv_map_size: i32) -> Result<usize, Error> {
         // When 0, we assume that map_size was filled by the user or const
         /* TODO autofill map size from the observer
 
@@ -1049,16 +1049,30 @@ where
             self.map_size = Some(fsrv_map_size as usize);
         }
         */
-        let mut map_size = fsrv_map_size;
-        if map_size % 64 != 0 {
-            map_size = ((map_size + 63) >> 6) << 6;
+        let mut actual_map_size = fsrv_map_size;
+        if actual_map_size % 64 != 0 {
+            actual_map_size = ((actual_map_size + 63) >> 6) << 6;
         }
 
         // TODO set AFL_MAP_SIZE
-        assert!(self.map_size.is_none() || map_size as usize <= self.map_size.unwrap());
+        if let Some(max_size) = self.map_size {
+            if actual_map_size as usize > max_size {
+                return Err(Error::illegal_state(format!(
+                    "The target map size is {actual_map_size} but the allocated map size is {max_size}. \
+                    Increase the initial size of the forkserver map to at least that size using the forkserver builder's `coverage_map_size`."
+            )));
+            }
+        } else {
+            return Err(Error::illegal_state(format!(
+                "The target map size is {actual_map_size} but we did not create a coverage map before launching the target! \
+                Set an initial forkserver map to at least that size using the forkserver builder's `coverage_map_size`."
+            )));
+        }
 
         // we'll use this later when we truncate the observer
-        self.map_size = Some(map_size as usize);
+        self.map_size = Some(actual_map_size as usize);
+
+        Ok(actual_map_size as usize)
     }
 
     /// Use autodict?