diff --git a/crates/functions/src/engineering.rs b/crates/functions/src/engineering.rs
index 0baf2b2ca..2899ad809 100644
--- a/crates/functions/src/engineering.rs
+++ b/crates/functions/src/engineering.rs
@@ -62,6 +62,15 @@ fn format_base(v: i64, radix: u32, places: Option<f64>) -> R<Value> {
     if v < min || v > max {
         return Err(CellError::Num);
     }
+    let places = places
+        .map(|p| {
+            let p = p.trunc();
+            if !(1.0..=10.0).contains(&p) {
+                return Err(CellError::Num);
+            }
+            Ok(p as usize)
+        })
+        .transpose()?;
     if v < 0 {
         let u = v + (1i64 << bits);
         return text_val(to_radix(u, radix));
@@ -70,11 +79,6 @@ fn format_base(v: i64, radix: u32, places: Option<f64>) -> R<Value> {
     match places {
         None => text_val(s),
         Some(p) => {
-            let p = p.trunc();
-            if !(1.0..=10.0).contains(&p) {
-                return Err(CellError::Num);
-            }
-            let p = p as usize;
             if s.len() > p {
                 return Err(CellError::Num);
             }
@@ -97,7 +101,13 @@ fn to_radix(mut v: i64, radix: u32) -> String {
 }
 
 fn places_arg(a: &[Arg]) -> R<Option<f64>> {
-    if has(a, 1) { Ok(Some(num(a, 1)?)) } else { Ok(None) }
+    if !has(a, 1) {
+        return Ok(None);
+    }
+    if matches!(a.get(1).map(scalar), Some(Value::Bool(_))) {
+        return Err(CellError::Value);
+    }
+    Ok(Some(num(a, 1)?))
 }
 
 fn convert_base(a: &[Arg], from: u32, to: u32) -> R<Value> {
@@ -402,6 +412,12 @@ mod tests {
         is_text(ev("BIN2OCT", vec![n(1111111111.0)]), "7777777777");
         is_text(ev("DEC2BIN", vec![n(9.0), n(4.0)]), "1001");
         is_text(ev("DEC2BIN", vec![n(-100.0)]), "1110011100");
+        is_err(ev("DEC2BIN", vec![n(-1.0), n(0.0)]), CellError::Num);
+        is_err(ev("DEC2HEX", vec![n(-1.0), n(11.0)]), CellError::Num);
+        is_err(ev("DEC2OCT", vec![n(-1.0), n(-1.0)]), CellError::Num);
+        is_err(ev("DEC2BIN", vec![n(1.0), crate::Arg::val(Value::Bool(true))]), CellError::Value);
+        is_text(ev("DEC2BIN", vec![n(-1.0), n(1.0)]), "1111111111");
+        is_text(ev("DEC2HEX", vec![n(-1.0), n(10.0)]), "FFFFFFFFFF");
         is_err(ev("DEC2BIN", vec![n(512.0)]), CellError::Num);
         is_text(ev("DEC2HEX", vec![n(100.0), n(4.0)]), "0064");
         is_text(ev("DEC2HEX", vec![n(-54.0)]), "FFFFFFFFCA");
diff --git a/docs/function-parity.md b/docs/function-parity.md
index 8fbfe52a5..ac5bd4cbb 100644
--- a/docs/function-parity.md
+++ b/docs/function-parity.md
@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (base conversion rejects surrounding whitespace in source strings, as observed in Excel; earlier: ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; IMSQRT reproduces Excel's stray real part for negative reals) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (base-conversion places are validated for negative inputs and reject Boolean arguments, as observed in Excel; earlier: base conversion rejects surrounding whitespace in source strings, as observed in Excel; earlier: ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; IMSQRT reproduces Excel's stray real part for negative reals) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Excel's worksheet functions, category by category, against GridCraft's function library. Part of
 [target-app-parity.md](target-app-parity.md); gaps feed [gaps.md](gaps.md).
@@ -125,6 +125,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 | Date | Change | Summary |
 |---|---|---|
 | 2026-10-31 | minor | BIN2*, HEX2* and OCT2* reject surrounding spaces and tabs in source strings with `#NUM!`, matching Excel; plain unpadded values remain valid |
+| 2026-10-31 | minor | DEC2BIN, DEC2HEX and DEC2OCT validate `places` before returning negative two's-complement output, and Boolean places return `#VALUE!`, matching Excel |
 | 2026-10-27 | minor | ECMA.CEILING added to the functions that are in our library but not on Microsoft's list (with USDOLLAR), so the counts don't change |
 | 2026-10-26 | minor | IMSQRT uses Excel's polar route with its cosine reduction, so `IMSQRT("-4")` is `1.22514845490862E-16+2i` as in Excel (exact `2i` before); the rows list the IM* differences still open |
 | 2026-10-25 | minor | LAMBDA row: a LAMBDA that calls itself more than 140 times, and a formula nested past the depth limit, give `#NUM!` (#1809; nesting gave `#CALC!` before). `=Foo(1)` where the defined name Foo holds `#N/A` gives `#N/A`, not `#NAME?` |