diff --git a/src/zr/playerclasses/apply.inc b/src/zr/playerclasses/apply.inc
index 5043268..6934400 100644
--- a/src/zr/playerclasses/apply.inc
+++ b/src/zr/playerclasses/apply.inc
@@ -74,6 +74,8 @@ bool:ClassApplyModel(client, classindex, cachetpye = ZR_CLASS_CACHE_PLAYER)
         Format(modelpath, sizeof(modelpath), "%s.mdl", modelpath);
     }
     
+    // TODO: Add support for keeping the default cs model ("default").
+    
     SetPlayerModel(client, modelpath);
     return true;
 }
diff --git a/src/zr/playerclasses/filtertools.inc b/src/zr/playerclasses/filtertools.inc
index e05daae..fa7b7bf 100644
--- a/src/zr/playerclasses/filtertools.inc
+++ b/src/zr/playerclasses/filtertools.inc
@@ -45,15 +45,138 @@ bool:ClassValidateTeamRequirements(cachetype = ZR_CLASS_CACHE_ORIGINAL)
 
 /**
  * Validates all the class attributes in the original class data array, to
- * check if they have invalid values.
+ * check if they have invalid values. Boolean settings are not validated.
  *
  * @param classindex    The index of the class to validate.
- * @return              True if validation was successful, false otherwise.
+ * @return              A value with attribute error flags.
  */
-bool:ClassValidateAttributes(classindex)
+ClassValidateAttributes(classindex)
 {
-    return true;
-    // TODO: Manually check each attribute (hard coded). Better solutions?
+    // TODO: Validate immunity mode and amount.
+    // TODO: Validate jump values.
+    
+    new flags;
+    
+    // Name.
+    if (strlen(ClassData[classindex][class_name]) == 0)
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_NAME;
+    }
+    
+    // Description.
+    if (strlen(ClassData[classindex][class_description]) == 0)
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_DESCRIPTION;
+    }
+    
+    // Model path.
+    decl String:model_path[256];
+    if (strcopy(model_path, sizeof(model_path), ClassData[classindex][class_model_path]) == 0)
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_MODEL_PATH;
+    }
+    else
+    {
+        // Check if default or random model is specified.
+        if (strcmp(model_path, "random", false) != 0 && strcmp(model_path, "default", false) != 0)
+        {
+            // Check if the file exists.
+            if (!FileExists(model_path))
+            {
+                flags += ZR_CLASS_ATTRIB_ERR_MODEL_PATH;
+            }
+        }        
+    }
+    
+    // Alpha, initial.
+    new alpha_initial = ClassData[classindex][class_alpha_initial];
+    if (!(alpha_initial >= 0 && alpha_initial <= 255))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_ALPHA_INITIAL;
+    }
+    
+    // Alpha, damaged.
+    new alpha_damaged = ClassData[classindex][class_alpha_damaged];
+    if (!(alpha_damaged >= 0 && alpha_damaged <= 255))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_ALPHA_DAMAGED;
+    }
+    
+    // Alpha, damage.
+    new alpha_damage = ClassData[classindex][class_alpha_damage];
+    if (!(alpha_damage >= 0 && alpha_damage <= 65536))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_ALPHA_DAMAGE;
+    }
+    
+    // Overlay path.
+    decl String:overlay_path[256];
+    if (strcopy(overlay_path, sizeof(overlay_path), ClassData[classindex][class_overlay_path]) != 0)
+    {
+        // Check if the file exists.
+        if (!FileExists(overlay_path))
+        {
+            flags += ZR_CLASS_ATTRIB_ERR_OVERLAY_PATH;
+        }
+    }
+    
+    // Field of view.
+    new fov = ClassData[classindex][class_fov];
+    if (!(fov > 15 && fov < 180))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_FOV;
+    }
+    
+    // Napalm time.
+    new Float:napalm_time = ClassData[classindex][class_napalm_time];
+    if (!(napalm_time >= 0.0 && napalm_time <= 900.0))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_NAPALM_TIME;
+    }
+    
+    // Health regen interval.
+    new Float:regen_interval = ClassData[classindex][class_health_regen_interval];
+    if (!(regen_interval >= 0.0 && regen_interval <= 900.0))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_HEALTH_REGEN_INTERVAL;
+    }
+    
+    // Health regen amount.
+    new regen_amount = ClassData[classindex][class_health_regen_amount];
+    if (!(regen_amount > 0 && regen_amount <= 65536))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_HEALTH_REGEN_AMOUNT;
+    }
+    
+    // Health infect gain.
+    new infect_gain = ClassData[classindex][class_health_infect_gain];
+    if (!(infect_gain >= 0 && infect_gain <= 65536))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_INFECT_GAIN;
+    }
+    
+    // Kill bonus.
+    new kill_bonus = ClassData[classindex][class_kill_bonus];
+    if (!(kill_bonus >= 0 && kill_bonus <= 128))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_KILL_BONUS;
+    }
+    
+    // Speed.
+    new Float:speed = ClassData[classindex][class_speed];
+    if (!(speed >= 0.0 && speed <= 1024.0))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_SPEED;
+    }
+    
+    // Knockback.
+    new Float:knockback = ClassData[classindex][class_knockback];
+    if (!(knockback >= -10.0 && knockback <= 50.0))
+    {
+        flags += ZR_CLASS_ATTRIB_ERR_KNOCKBACK;
+    }
+    
+    return flags;
 }
 
 /**
@@ -65,12 +188,7 @@ bool:ClassValidateAttributes(classindex)
  */
 bool:ClassValidateIndex(classindex)
 {
-    if (ClassCount == 0)
-    {
-        return false;
-    }
-    
-    if (classindex >= 0 && classid < ClassCount)
+    if (classindex >= 0 && classindex < ClassCount)
     {
         return true;
     }
diff --git a/src/zr/playerclasses/playerclasses.inc b/src/zr/playerclasses/playerclasses.inc
index 7b0b148..2509331 100644
--- a/src/zr/playerclasses/playerclasses.inc
+++ b/src/zr/playerclasses/playerclasses.inc
@@ -129,6 +129,34 @@
  * @endsection
  */
 
+/**
+ * @section Error flags for invalid class attributes.
+ */
+#define ZR_CLASS_ATTRIB_ERR_OK                      0
+#define ZR_CLASS_ATTRIB_ERR_NAME                    1
+#define ZR_CLASS_ATTRIB_ERR_DESCRIPTION             2
+#define ZR_CLASS_ATTRIB_ERR_MODEL_PATH              4
+#define ZR_CLASS_ATTRIB_ERR_ALPHA_INITIAL           8
+#define ZR_CLASS_ATTRIB_ERR_ALPHA_DAMAGED           16
+#define ZR_CLASS_ATTRIB_ERR_ALPHA_DAMAGE            32
+#define ZR_CLASS_ATTRIB_ERR_OVERLAY_PATH            64
+#define ZR_CLASS_ATTRIB_ERR_FOV                     128
+#define ZR_CLASS_ATTRIB_ERR_NAPALM_TIME             256
+#define ZR_CLASS_ATTRIB_ERR_IMMUNITY_MODE           512
+#define ZR_CLASS_ATTRIB_ERR_IMMUNITY_AMOUNT         1024
+#define ZR_CLASS_ATTRIB_ERR_HEALTH_REGEN_INTERVAL   2048
+#define ZR_CLASS_ATTRIB_ERR_HEALTH_REGEN_AMOUNT     4096
+#define ZR_CLASS_ATTRIB_ERR_INFECT_GAIN             8192
+#define ZR_CLASS_ATTRIB_ERR_KILL_BONUS              16384
+#define ZR_CLASS_ATTRIB_ERR_SPEED                   32768
+#define ZR_CLASS_ATTRIB_ERR_KNOCKBACK               65536
+#define ZR_CLASS_ATTRIB_ERR_JUMP_HEIGHT             131072
+#define ZR_CLASS_ATTRIB_ERR_JUMP_DISTANCE           262144
+/**
+ * @endsection
+ */
+
+
 /**
  * Generic player attributes.
  */
@@ -249,6 +277,7 @@ ClassLoad(const String:classfile[256] = "configs/zr/playerclasses.txt")
     decl String:overlay_path[256];
     
     ClassCount = 0;
+    new ClassErrorFlags;
     
     // Loop through all classes and store attributes in the ClassData array.
     do
@@ -313,17 +342,20 @@ ClassLoad(const String:classfile[256] = "configs/zr/playerclasses.txt")
         ClassData[ClassCount][class_jump_distance] = KvGetFloat(kvClassData, "jump_distance", ZR_CLASS_DEFAULT_JUMP_DISTANCE);
         
         // Validate the class attributes.
-        if (!ClassValidateAttributes(ClassCount))
+        ClassErrorFlags = ClassValidateAttributes(ClassCount);
+        if (ClassErrorFlags > 0)
         {
-            // TODO: There's an invalid class, what do we do?
-            // Skip it (clearing data too), disable it and give a log warning,
-            // or set the plugin in a failed state?
-        }
-        else
-        {
-            // The class is valid. Update the counter.
-            ClassCount++;
+            // There's one or more invalid class attributes. Disable the class
+            // and log an error message.
+            ClassData[ClassCount][class_enabled] = false;
+            if (LogFlagCheck(LOG_CORE_EVENTS, LOG_MODULE_CLASSES))
+            {
+                ZR_LogMessageFormatted(-1, "classes", "load", "Invalid class at index %d. Class error flags: %d.", LOG_FORMAT_TYPE_ERROR, ClassCount, ClassErrorFlags);
+            }
         }
+
+        // Update the counter.
+        ClassCount++;
     } while (KvGotoNextKey(kvClassData));
     
     // Validate team requirements.