diff --git a/cmd/readvar/src/lib.rs b/cmd/readvar/src/lib.rs
index 4d077a814..f7781058e 100644
--- a/cmd/readvar/src/lib.rs
+++ b/cmd/readvar/src/lib.rs
@@ -135,14 +135,19 @@ fn readvar(context: &mut ExecutionContext) -> Result<()> {
         n == v || n.ends_with(&suffix)
     }
 
-    if let Some(ref variable) = subargs.variable {
+    if let Some(variable) = &subargs.variable {
         let m =
             if variable.contains("::") { match_exact } else { match_suffix };
 
+        let mut found = false;
         for (n, v) in
             hubris.qualified_variables().filter(|&(n, _)| m(n, variable))
         {
             readvar_dump(hubris, core, v, n, &subargs)?;
+            found = true;
+        }
+        if !found {
+            bail!("variable '{variable}' not found; use \"-l\" to list");
         }
     } else {
         bail!("expected variable (use \"-l\" to list)");
diff --git a/humility-core/src/core.rs b/humility-core/src/core.rs
index 66c27444b..765650397 100644
--- a/humility-core/src/core.rs
+++ b/humility-core/src/core.rs
@@ -148,47 +148,47 @@ impl Core for UnattachedCore {
     }
 
     fn read_8(&mut self, _addr: u32, _data: &mut [u8]) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::read_8 unimplemented when unattached!");
     }
 
     fn read_reg(&mut self, _reg: ARMRegister) -> Result<u32> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::read_reg unimplemented when unattached!");
     }
 
     fn write_reg(&mut self, _reg: ARMRegister, _value: u32) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::write_reg unimplemented when unattached!");
     }
 
     fn write_word_32(&mut self, _addr: u32, _data: u32) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::write_word_32 unimplemented when unattached!");
     }
 
     fn write_8(&mut self, _addr: u32, _data: &[u8]) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::write_8 unimplemented when unattached!");
     }
 
     fn halt(&mut self) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::halt unimplemented when unattached!");
     }
 
     fn run(&mut self) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::run unimplemented when unattached!");
     }
 
     fn step(&mut self) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::step unimplemented when unattached!");
     }
 
     fn init_swv(&mut self) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::init_swv unimplemented when unattached!");
     }
 
     fn read_swv(&mut self) -> Result<Vec<u8>> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::read_swv unimplemented when unattached!");
     }
 
     fn load(&mut self, _path: &Path) -> Result<()> {
-        bail!("Unimplemented when unattached!");
+        bail!("Core::load unimplemented when unattached!");
     }
 
     fn reset(&mut self) -> Result<()> {
diff --git a/humility-net-core/src/lib.rs b/humility-net-core/src/lib.rs
index 6b044774c..42654b914 100644
--- a/humility-net-core/src/lib.rs
+++ b/humility-net-core/src/lib.rs
@@ -221,7 +221,8 @@ impl NetCore {
             // least one task associated with it.  If this task is the
             // supervisor, then that's bad news.
             let region = ram[p - 1].clone();
-            let Some(&task) = region.tasks.iter().find(|t| t.task() != 0) else {
+            let Some(&task) = region.tasks.iter().find(|t| t.task() != 0)
+            else {
                 bail!("supervisor memory cannot be read using dump facilities");
             };
 
diff --git a/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.stderr b/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.stderr
index da2ede4a0..c37c83ff5 100644
--- a/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.stderr
+++ b/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.stderr
@@ -1 +1,2 @@
 humility: attached to dump
+humility readvar failed: variable 'TICKS' not found; use "-l" to list
diff --git a/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.toml b/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.toml
index e1c38e887..2789b8362 100644
--- a/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.toml
+++ b/tests/cmd/readvar-ticks/readvar-ticks.extern-regions.toml
@@ -7,4 +7,4 @@
 fs.base = "../cores"
 bin.name = "humility"
 args = "-d hubris.core.extern-regions readvar TICKS"
-
+status.code = 1 # Hand-edit: no TICKS variable is present in this image
diff --git a/tests/cmd/readvar-ticks/readvar-ticks.task.net.stderr b/tests/cmd/readvar-ticks/readvar-ticks.task.net.stderr
index da2ede4a0..c37c83ff5 100644
--- a/tests/cmd/readvar-ticks/readvar-ticks.task.net.stderr
+++ b/tests/cmd/readvar-ticks/readvar-ticks.task.net.stderr
@@ -1 +1,2 @@
 humility: attached to dump
+humility readvar failed: variable 'TICKS' not found; use "-l" to list
diff --git a/tests/cmd/readvar-ticks/readvar-ticks.task.net.toml b/tests/cmd/readvar-ticks/readvar-ticks.task.net.toml
index 37c27a5ba..cd296c8f2 100644
--- a/tests/cmd/readvar-ticks/readvar-ticks.task.net.toml
+++ b/tests/cmd/readvar-ticks/readvar-ticks.task.net.toml
@@ -7,4 +7,4 @@
 fs.base = "../cores"
 bin.name = "humility"
 args = "-d hubris.core.task.net readvar TICKS"
-
+status.code = 1 # Hand-edit: no TICKS variable is present in this image