diff --git a/doc/developer-notes.md b/doc/developer-notes.md
--- a/doc/developer-notes.md
+++ b/doc/developer-notes.md
@@ -487,6 +487,12 @@
   - *Rationale*: Ensure determinism by avoiding accidental use of uninitialized
     values. Also, static analyzers balk about this.
 
+- By default, declare single-argument constructors `explicit`.
+
+  - *Rationale*: This is a precaution to avoid unintended conversions that might
+    arise when single-argument constructors are used as implicit conversion
+    functions.
+
 - Use explicitly signed or unsigned `char`s, or even better `uint8_t` and
   `int8_t`. Do not use bare `char` unless it is to pass to a third-party API.
   This type can be signed or unsigned depending on the architecture, which can
diff --git a/src/test/addrman_tests.cpp b/src/test/addrman_tests.cpp
--- a/src/test/addrman_tests.cpp
+++ b/src/test/addrman_tests.cpp
@@ -17,7 +17,7 @@
     uint64_t state;
 
 public:
-    CAddrManTest(bool makeDeterministic = true) {
+    explicit CAddrManTest(bool makeDeterministic = true) {
         state = 1;
 
         if (makeDeterministic) {