diff --git a/src/io/nayuki/fastqrcodegen/BitBuffer.java b/src/io/nayuki/fastqrcodegen/BitBuffer.java
index a2d573e..f187e9f 100644
--- a/src/io/nayuki/fastqrcodegen/BitBuffer.java
+++ b/src/io/nayuki/fastqrcodegen/BitBuffer.java
@@ -87,6 +87,8 @@ final class BitBuffer {
 	public void appendBits(int val, int len) {
 		if (len < 0 || len > 31 || val >>> len != 0)
 			throw new IllegalArgumentException("Value out of range");
+		if (Integer.MAX_VALUE - bitLength < len)
+			throw new IllegalStateException("Maximum length reached");
 		
 		if (bitLength + len + 1 > data.length << 5)
 			data = Arrays.copyOf(data, data.length * 2);
diff --git a/src/io/nayuki/fastqrcodegen/QrCode.java b/src/io/nayuki/fastqrcodegen/QrCode.java
index 42fdaec..4366d5f 100644
--- a/src/io/nayuki/fastqrcodegen/QrCode.java
+++ b/src/io/nayuki/fastqrcodegen/QrCode.java
@@ -381,6 +381,7 @@ public final class QrCode {
 	// Returns a new byte string representing the given data with the appropriate error correction
 	// codewords appended to it, based on this object's version and error correction level.
 	private byte[] addEccAndInterleave(byte[] data) {
+		Objects.requireNonNull(data);
 		if (data.length != getNumDataCodewords(version, errorCorrectionLevel))
 			throw new IllegalArgumentException();
 		
diff --git a/src/io/nayuki/fastqrcodegen/QrSegment.java b/src/io/nayuki/fastqrcodegen/QrSegment.java
index ee6a6a2..9f91ac4 100644
--- a/src/io/nayuki/fastqrcodegen/QrSegment.java
+++ b/src/io/nayuki/fastqrcodegen/QrSegment.java
@@ -146,12 +146,14 @@ public final class QrSegment {
 	 */
 	public static QrSegment makeEci(int assignVal) {
 		BitBuffer bb = new BitBuffer();
-		if (0 <= assignVal && assignVal < (1 << 7))
+		if (assignVal < 0)
+			throw new IllegalArgumentException("ECI assignment value out of range");
+		else if (assignVal < (1 << 7))
 			bb.appendBits(assignVal, 8);
-		else if ((1 << 7) <= assignVal && assignVal < (1 << 14)) {
+		else if (assignVal < (1 << 14)) {
 			bb.appendBits(2, 2);
 			bb.appendBits(assignVal, 14);
-		} else if ((1 << 14) <= assignVal && assignVal < 1000000) {
+		} else if (assignVal < 1_000_000) {
 			bb.appendBits(6, 3);
 			bb.appendBits(assignVal, 21);
 		} else
@@ -284,7 +286,7 @@ public final class QrSegment {
 		/*-- Constructor --*/
 		
 		private Mode(int mode, int... ccbits) {
-			this.modeBits = mode;
+			modeBits = mode;
 			numBitsCharCount = ccbits;
 		}
 		
@@ -294,6 +296,7 @@ public final class QrSegment {
 		// Returns the bit width of the character count field for a segment in this mode
 		// in a QR Code at the given version number. The result is in the range [0, 16].
 		int numCharCountBits(int ver) {
+			assert QrCode.MIN_VERSION <= ver && ver <= QrCode.MAX_VERSION;
 			return numBitsCharCount[(ver + 7) / 17];
 		}