diff --git a/pythainlp/khavee/core.py b/pythainlp/khavee/core.py
index f940e8922..7e2f78f80 100644
--- a/pythainlp/khavee/core.py
+++ b/pythainlp/khavee/core.py
@@ -54,9 +54,15 @@ def check_sara(self, word: str) -> str:
         sara = []
         countoa = 0
 
+        if not word:
+            return ""
+
         # In case of การันย์
         if "์" in word[-1]:
             word = word[:-2]
+            # After removing the karun, the word may become empty (e.g. "ก์")
+            if not word:
+                return ""
 
         # In case of สระเดี่ยว
         for i in word:
@@ -253,6 +259,9 @@ def check_marttra(self, word: str) -> str:
         word = self.handle_karun_sound_silence(word)
         word = remove_tonemark(word)
 
+        if not word:
+            return ""
+
         # Check for ำ at the end (represents "am" sound, ends with m)
         if word[-1] == "ำ":
             return "กม"
diff --git a/pythainlp/morpheme/word_formation.py b/pythainlp/morpheme/word_formation.py
index 63453a25e..45817dc31 100644
--- a/pythainlp/morpheme/word_formation.py
+++ b/pythainlp/morpheme/word_formation.py
@@ -31,6 +31,14 @@ def nighit(w1: str, w2: str) -> str:
         assert nighit("สํ","ปทา")=="สัมปทา"
         assert nighit("สํ","โยค")=="สังโยค"
     """
+    if not isinstance(w1, str) or not isinstance(w2, str):
+        raise TypeError("Both w1 and w2 must be strings.")
+    w1 = w1.strip()
+    w2 = w2.strip()
+    if not w1:
+        return w2
+    if not w2:
+        return w1
     if not str(w1).endswith("ํ") and len(w1) != 2:
         raise NotImplementedError(f"The function doesn't support {w1}.")
     list_w1 = list(w1)
@@ -38,7 +46,13 @@ def nighit(w1: str, w2: str) -> str:
     newword = []
     newword.append(list_w1[0])
     newword.append("ั")
-    consonant_start = [i for i in list_w2 if i in set(thai_consonants)][0]
+    _consonants = set(thai_consonants)
+    consonants_in_w2 = [i for i in list_w2 if i in _consonants]
+    if not consonants_in_w2:
+        raise ValueError(
+            f"w2 {w2!r} contains no Thai consonants."
+        )
+    consonant_start = consonants_in_w2[0]
     if consonant_start in ["ก", "ช", "ค", "ข", "ง"]:
         newword.append("ง")
     elif consonant_start in ["จ", "ฉ", "ช", "ฌ"]: