diff --git a/docs/v-client-library-docs.rst b/docs/v-client-library-docs.rst
index b35f15d..2e4187c 100644
--- a/docs/v-client-library-docs.rst
+++ b/docs/v-client-library-docs.rst
@@ -334,9 +334,10 @@ enum Boolean
 
    pub enum Boolean {
    	// These must not be negative values because they are encoded as u8 on disk.
-   	is_unknown = 0 // same as NULL
-   	is_false   = 1
-   	is_true    = 2
+   	// 0 is resevered for encoding NULL on disk, but is not a valid value in
+   	// memory.
+   	is_false = 1
+   	is_true  = 2
    }
 
 
diff --git a/tests/boolean.sql b/tests/boolean.sql
index 01958ea..10d847f 100644
--- a/tests/boolean.sql
+++ b/tests/boolean.sql
@@ -111,7 +111,7 @@ VALUES FALSE IS NULL;
 -- COL1: FALSE
 
 VALUES UNKNOWN IS NULL;
--- COL1: FALSE
+-- COL1: TRUE
 
 VALUES TRUE IS NOT TRUE;
 -- COL1: FALSE
@@ -147,7 +147,7 @@ VALUES FALSE IS NOT NULL;
 -- COL1: TRUE
 
 VALUES UNKNOWN IS NOT NULL;
--- COL1: TRUE
+-- COL1: FALSE
 
 VALUES NOT TRUE;
 -- COL1: FALSE
diff --git a/vsql/eval.v b/vsql/eval.v
index ad15387..430c425 100644
--- a/vsql/eval.v
+++ b/vsql/eval.v
@@ -437,26 +437,18 @@ fn eval_cast(mut conn Connection, data Row, e CastExpr, params map[string]Value)
 }
 
 fn eval_truth(mut conn Connection, data Row, e TruthExpr, params map[string]Value) !Value {
+	// See ISO/IEC 9075-2:2016(E), 6.39, <boolean value expression>,
+	// "Table 15 — Truth table for the IS boolean operator"
+
 	value := eval_as_value(mut conn, data, e.expr, params)!
-	result := match value.bool_value() {
-		.is_true {
-			match e.value.bool_value() {
-				.is_true { new_boolean_value(true) }
-				.is_false, .is_unknown { new_boolean_value(false) }
-			}
-		}
-		.is_false {
-			match e.value.bool_value() {
-				.is_true, .is_unknown { new_boolean_value(false) }
-				.is_false { new_boolean_value(true) }
-			}
-		}
-		.is_unknown {
-			match e.value.bool_value() {
-				.is_true, .is_false { new_boolean_value(false) }
-				.is_unknown { new_boolean_value(true) }
-			}
-		}
+	mut result := new_boolean_value(false)
+
+	if value.is_null {
+		result = new_boolean_value(e.value.is_null)
+	} else if value.bool_value() == .is_true {
+		result = new_boolean_value(e.value.bool_value() == .is_true)
+	} else {
+		result = new_boolean_value(e.value.bool_value() == .is_false)
 	}
 
 	if e.not {
diff --git a/vsql/operators.v b/vsql/operators.v
index b404981..caba49f 100644
--- a/vsql/operators.v
+++ b/vsql/operators.v
@@ -157,10 +157,13 @@ fn unary_negate_double_precision(conn &Connection, v Value) !Value {
 }
 
 fn unary_not_boolean(conn &Connection, v Value) !Value {
+	if v.is_null {
+		return new_unknown_value()
+	}
+
 	return match v.bool_value() {
 		.is_true { new_boolean_value(false) }
 		.is_false { new_boolean_value(true) }
-		.is_unknown { new_unknown_value() }
 	}
 }
 
@@ -229,47 +232,41 @@ fn binary_varchar_concat_varchar(conn &Connection, a Value, b Value) !Value {
 }
 
 fn binary_boolean_and_boolean(conn &Connection, a Value, b Value) !Value {
-	match a.bool_value() {
-		.is_true {
-			return match b.bool_value() {
-				.is_true { new_boolean_value(true) }
-				.is_false { new_boolean_value(false) }
-				.is_unknown { new_unknown_value() }
-			}
-		}
-		.is_false {
+	// See ISO/IEC 9075-2:2016(E), 6.39, <boolean value expression>,
+	// "Table 13 — Truth table for the AND boolean operator"
+
+	if a.is_null {
+		if b.bool_value() == .is_false {
 			return new_boolean_value(false)
 		}
-		.is_unknown {
-			return match b.bool_value() {
-				.is_true { new_unknown_value() }
-				.is_false { new_boolean_value(false) }
-				.is_unknown { new_unknown_value() }
-			}
-		}
+
+		return new_unknown_value()
+	}
+
+	if a.bool_value() == .is_true {
+		return b
 	}
+
+	return new_boolean_value(false)
 }
 
 fn binary_boolean_or_boolean(conn &Connection, a Value, b Value) !Value {
-	match a.bool_value() {
-		.is_true {
+	// See ISO/IEC 9075-2:2016(E), 6.39, <boolean value expression>,
+	// "Table 13 — Truth table for the AND boolean operator"
+
+	if a.is_null {
+		if b.bool_value() == .is_true {
 			return new_boolean_value(true)
 		}
-		.is_false {
-			return match b.bool_value() {
-				.is_true { new_boolean_value(true) }
-				.is_false { new_boolean_value(false) }
-				.is_unknown { new_unknown_value() }
-			}
-		}
-		.is_unknown {
-			return match b.bool_value() {
-				.is_true { new_boolean_value(true) }
-				.is_false { new_unknown_value() }
-				.is_unknown { new_unknown_value() }
-			}
-		}
+
+		return new_unknown_value()
+	}
+
+	if a.bool_value() == .is_true {
+		return new_boolean_value(true)
 	}
+
+	return b
 }
 
 fn binary_float_equal_float(conn &Connection, a Value, b Value) !Value {
diff --git a/vsql/row.v b/vsql/row.v
index 4ef03fc..3779fa4 100644
--- a/vsql/row.v
+++ b/vsql/row.v
@@ -280,9 +280,11 @@ fn new_row_from_bytes(t Table, data []u8, tid int) Row {
 			match col.typ.typ {
 				.is_boolean {
 					unsafe {
-						v.v.bool_value = Boolean(buf.read_u8())
-						if v.bool_value() == .is_unknown {
+						b := buf.read_u8()
+						if b == 0 {
 							v.is_null = true
+						} else {
+							v.v.bool_value = Boolean(b)
 						}
 					}
 				}
diff --git a/vsql/value.v b/vsql/value.v
index 4c48122..e107be7 100644
--- a/vsql/value.v
+++ b/vsql/value.v
@@ -11,9 +11,10 @@ import regex
 // Possible values for a BOOLEAN.
 pub enum Boolean {
 	// These must not be negative values because they are encoded as u8 on disk.
-	is_unknown = 0 // same as NULL
-	is_false   = 1
-	is_true    = 2
+	// 0 is resevered for encoding NULL on disk, but is not a valid value in
+	// memory.
+	is_false = 1
+	is_true  = 2
 }
 
 // Returns ``TRUE``, ``FALSE`` or ``UNKNOWN``.
@@ -21,7 +22,6 @@ pub fn (b Boolean) str() string {
 	return match b {
 		.is_false { 'FALSE' }
 		.is_true { 'TRUE' }
-		.is_unknown { 'UNKNOWN' }
 	}
 }
 
@@ -80,9 +80,7 @@ pub fn new_boolean_value(b bool) Value {
 pub fn new_unknown_value() Value {
 	return Value{
 		typ: Type{.is_boolean, 0, 0, false}
-		v: InternalValue{
-			bool_value: .is_unknown
-		}
+		is_null: true
 	}
 }
 
@@ -286,8 +284,8 @@ fn (v Value) as_numeric() !big.Integer {
 // The string representation of this value. Different types will have different
 // formatting.
 pub fn (v Value) str() string {
-	if v.is_null && v.typ.typ != .is_boolean {
-		return 'NULL'
+	if v.is_null {
+		return if v.typ.typ == .is_boolean { 'UNKNOWN' } else { 'NULL' }
 	}
 
 	return match v.typ.typ {