diff --git a/c/qrcodegen.c b/c/qrcodegen.c
index 1c06546..8d53419 100644
--- a/c/qrcodegen.c
+++ b/c/qrcodegen.c
@@ -704,7 +704,9 @@ static long getPenaltyScore(const uint8_t qrcode[]) {
 	int total = qrsize * qrsize;  // Note that size is odd, so dark/total != 1/2
 	// Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 	int k = (int)((labs(dark * 20L - total * 10L) + total - 1) / total) - 1;
+	assert(0 <= k && k <= 9);
 	result += k * PENALTY_N4;
+	assert(0 <= result && result <= 2568888L);  // Non-tight upper bound based on default values of PENALTY_N1, ..., N4
 	return result;
 }
 
diff --git a/cpp/qrcodegen.cpp b/cpp/qrcodegen.cpp
index 081b19c..b0f02af 100644
--- a/cpp/qrcodegen.cpp
+++ b/cpp/qrcodegen.cpp
@@ -663,7 +663,11 @@ long QrCode::getPenaltyScore() const {
 	int total = size * size;  // Note that size is odd, so dark/total != 1/2
 	// Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 	int k = static_cast<int>((std::abs(dark * 20L - total * 10L) + total - 1) / total) - 1;
+	if (!(0 <= k && k <= 9))
+		throw std::logic_error("Assertion error");
 	result += k * PENALTY_N4;
+	if (!(0 <= result && result <= 2568888L))  // Non-tight upper bound based on default values of PENALTY_N1, ..., N4
+		throw std::logic_error("Assertion error");
 	return result;
 }
 
diff --git a/java/src/main/java/io/nayuki/qrcodegen/QrCode.java b/java/src/main/java/io/nayuki/qrcodegen/QrCode.java
index 47edbdc..46f5d75 100644
--- a/java/src/main/java/io/nayuki/qrcodegen/QrCode.java
+++ b/java/src/main/java/io/nayuki/qrcodegen/QrCode.java
@@ -594,7 +594,9 @@ public final class QrCode {
 		int total = size * size;  // Note that size is odd, so dark/total != 1/2
 		// Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 		int k = (Math.abs(dark * 20 - total * 10) + total - 1) / total - 1;
+		assert 0 <= k && k <= 9;
 		result += k * PENALTY_N4;
+		assert 0 <= result && result <= 2568888;  // Non-tight upper bound based on default values of PENALTY_N1, ..., N4
 		return result;
 	}
 	
diff --git a/python/qrcodegen.py b/python/qrcodegen.py
index 254238c..3917456 100644
--- a/python/qrcodegen.py
+++ b/python/qrcodegen.py
@@ -470,7 +470,9 @@ class QrCode:
 		total: int = size**2  # Note that size is odd, so dark/total != 1/2
 		# Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 		k: int = (abs(dark * 20 - total * 10) + total - 1) // total - 1
+		assert 0 <= k <= 9
 		result += k * QrCode._PENALTY_N4
+		assert 0 <= result <= 2568888  # Non-tight upper bound based on default values of PENALTY_N1, ..., N4
 		return result
 	
 	
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index feb1947..2f6c713 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -678,7 +678,9 @@ impl QrCode {
 		let total: i32 = size * size;  // Note that size is odd, so dark/total != 1/2
 		// Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 		let k: i32 = ((dark * 20 - total * 10).abs() + total - 1) / total - 1;
+		assert!(0 <= k && k <= 9);
 		result += k * PENALTY_N4;
+		assert!(0 <= result && result <= 2568888);  // Non-tight upper bound based on default values of PENALTY_N1, ..., N4
 		result
 	}
 	
diff --git a/typescript-javascript/qrcodegen.ts b/typescript-javascript/qrcodegen.ts
index e0426fc..036fc9e 100644
--- a/typescript-javascript/qrcodegen.ts
+++ b/typescript-javascript/qrcodegen.ts
@@ -531,7 +531,11 @@ namespace qrcodegen {
 			const total: int = this.size * this.size;  // Note that size is odd, so dark/total != 1/2
 			// Compute the smallest integer k >= 0 such that (45-5k)% <= dark/total <= (55+5k)%
 			const k: int = Math.ceil(Math.abs(dark * 20 - total * 10) / total) - 1;
+			if (!(0 <= k && k <= 9))
+				throw "Assertion error";
 			result += k * QrCode.PENALTY_N4;
+			if (!(0 <= result && result <= 2568888))  // Non-tight upper bound based on default values of PENALTY_N1, ..., N4
+				throw "Assertion error";
 			return result;
 		}